Mechanical keyboards come in all shapes and sizes, many of which you can find on Amazon or via some other retailer. If you need a new board, buying a pre-built one is the cheapest and easiest way. However, building a custom keyboard gives you the chance to choose everything from the case material, to the switches, to the keycaps.
The popularity of custom keyboards has exploded in the last few years, making it a confusing and intimidating hobby to pick up. Let's break it all down.
Layouts and firmware
One of the things you'll notice about custom keyboards rather quickly is they tend to have unusual layouts, and they're often tiny compared to the standard full-sized 104-key layout. There are tenkeyless (80%) boards that lack a number pad, but also 65%, 60%, and smaller. A 60% is fairly common these days—these boards have only the main alphas, number row, and modifiers. The arrows and other keys are accessible via a function layer. A 65% board adds back the arrows and a few extra keys, but 40% boards go the other way with the alpha keys and a just a few modifiers. Then there are various split and ergonomic boards, like the Ergodox.
Some of these are available as niche pre-built keyboards, but there's one main difference between those and a truly custom board. A custom board is programmable, meaning you can have any of the keys do whatever you want. This is extremely important when you're dealing with fewer physical keys because you will need at least one robust function layer to fit in all the standard keyboard commands.
The firmware on a custom keyboard offers much more power than the desktop clients many fancy "gamer" keyboards use. After a board is programmed with your preferred layout, it doesn't rely on any software on a computer. It works exactly the same no matter which device you plug it into. The things you can do are also much more advanced. Some boards include advanced macro support or the option to control the mouse cursor.
A smaller keyboard layout can be much more efficient than a full sized one. By relegating some commands to a function layer, your hands don't have to move as far while typing, and your mouse stays closer to your hands. True, some people can't get by without a full layout and number pad, but most people who think they do are wrong. It's much easier to scale back the size of your board than you think.
The vast majority of switches compatible with custom boards are Cherry or Cherry clones. The term "clones" has a negative connotation, which isn't fair. Some of the clones are actually better than current Cherry switches, at least in my opinion.
You can still use Cherry switches in a custom board if you want—they come in linear, tactile, and clicky varieties. Then, there are different spring weights for each. As an example, there are clicky blue switches with a lighter spring weight. If you want something heavier, there are greens that also click. The divisions are similar in clone switches, and even the "color coding" of the stems might be the same.
A switch maker you'll see come up a lot in custom keyboard circles is Gateron, and for good reason. Its switches are better than Cherry in many ways. They come in more weights, and the action is noticeably smoother. Zealios are also a big deal these days (they're one of my favorite switches). These custom switches were designed by ZealPC and are manufactured by Gateron. They come in various spring weights with a strong tactile bump. For the most part, Cherry-style switches from Kailh, Zorro, Outemu, and others are not as good.
Since keyboard switches are good for millions of keystrokes, you can even salvage switches from boards that were built decades ago. Vintage Cherry Black switches are much sought after, for example. There are also non-Cherry switches made by Alps Electronics. These haven't been manufactured for years, but some people still prefer them to Cherry. There are even Alps clones you can get from a company called Matias. Alps switches aren't compatible with as many keycaps or PCBs, but the feel is unique.
So you bought some switches. Great, but you can do more than just buy them. You can also open those switches up and customize them. Many enthusiasts will apply lubricant to parts of the switch mechanism to change the feel. You can also swap parts of the housing or even the stem and spring to make it a completely different switch. For example, you can swap the stem from a super-clicky Aristotle Cherry clone into a Zealio housing to get a very smooth, clicky hybrid switch called a Zealiostotle.
The variety of switches and the ease with which you can modify them can make your board truly unique.
Some custom keyboards will come with a set of keycaps, often as an optional add-on. They aren't usually anything special, though. For maximum customization of your build, you may want to pick up a fancy custom keyset.
To pick the right caps, you'll need to decide what sort of profile you want to type on. The keycaps you see on mainstream boards are in OEM profile, which isn't very common in custom boards. The most similar to OEM is Cherry profile—it's lower and a bit more angled, though. These are available from a few manufacturers, the most well known being GMK. More on that shortly.
SA are taller keycaps that mimic the high-profile retro caps on IBM's beamspring mainframe keyboards. Only a few companies have the molds to make these, and the wait times are getting very long. For lower profile, there are DSA and XDA caps. Unlike the others, these profiles are not sculpted differently from one row to the next. That makes it easier to cover boards with unusual layouts, but not everyone likes typing on these flatter keycaps.
A set of custom keycaps could set you back $150-200, and one reason for that is simply that they're custom designed and produced in small batches. However, the quality of the materials is also a big step-up from what you're probably used to. These sets are usually done in either PBT or ABS plastic. PBT caps are extremely durable and the legends are "dye-sublimated." That means the dye is embedded in the plastic, so it never wears off. ABS caps are usually doubleshot, which means there are composed of two different molds, one with the legend on a lattice and another that fills in the rest of the keycap shape. That allows for awesome color combinations and legends that never fade.
Speaking of colors, custom sets are offered in many different colorways. Some of the most popular are Carbon, Hyperfuse, and Dolch. You can't just go to Amazon and buy them, though. You have to wait for someone to organize a sale, so you should know a few things about the various manufacturers.
GMK is famous for having extremely thick ABS doubleshot caps, which makes them feel extremely solid. Typing on them is a totally different experience if you've been using than ABS caps with lasered legends all these years. Other manufacturers like Signature Plastics produce good doubleshot caps in DSA and SA profile, but they're not quite as thick. PBT caps are also thicker, but that varies by manufacturer.
Then there's the dangerous world of artisan keycaps, small resin caps usually made from hand-sculpted molds. There are turtles, blowfish, skulls, robots, flowers, and so much more. Many artisans are colored to match popular keysets, but it's hard to find the right ones. The people who make artisan caps only have a limited supply, so you have to join raffles in order to even have the chance to spend $20-50 on a single cap. If you think that's crazy, stay away from the secondary market. A cap that you buy for $50 could go for several times more than that if you resell it.
Buying components: Group buys vs. pre-order
So, you've decided you want to build a keyboard with your ideal layout and switches tuned perfectly to your fingers. Great, now you get to wait, spend a lot of money, and wait some more. There are very few keyboard kits you can get on-demand from retailers. There are more keysets available, but the best ones are still hard to come by. The problem is these are niche products, and it costs money to keep them in stock constantly. The solution is "group buys," which are sort of like pre-orders… but not really.
When you join a group buy, you're essentially paying money to an organizer that pays a vendor or manufacturer to produce the group's items. The upshot, you tend to get things cheaper than retail cost. This is the part that makes it feel like a pre-order from your perspective—you buy something and have to wait a few months for it to arrive.
The difference here is who is taking on the risk. With a pre-order, it's the retailer. They commit to order X of a product. They pay for it, then have to sell it all to make their money. With a group buy, the group is assuming the risk. The organizer is just running things, and it's your money that's on the line. Group buys can take a long time, and even fail entirely in rare instances.
Group buys have traditionally been organized by community members on forums like Geekhack or Deskthority. There's also the Reddit /r/MechanicalKeyboards running some. However, some small outfits have appeared to add a bit more structure to the group buy.
Some of the more popular places to pick up components and kits include Keyclack and Originative. Then we have Massdrop, which has become a hub for group buys of all types, not just keyboards. They have support reps, community managers, and good relationships with various companies. Massdrop feels more like a retailer, but it's still running group buys.
If you don't want to wait on a group buy, there are precious few keyboard components available. Most of them are Chinese import sites like AliExpress. There are a few places you can get keycaps without waiting, including Originative and Pimp My Keyboard, which is the consumer storefront for Signature Plastics.
What to expect when building
In addition to the parts, you also need the proper tools to build a keyboard. Even the simpler kits require a soldering iron, which takes some practice to wield effectively. You'll also need some rosin core solder; the easiest to work with a 60/40 tin-lead mixture. Each switch you solder into a PCB has two points that need to be connected. Then add two more if you are adding an in-switch LED.
If you get a kit that has multiple possible layouts, make certain you have the switches plugged into the right spot. Try using keycaps to space them out correctly. This is especially important on the bottom row, which usually has many different configurations on universal boards.
Take the first few switches slow, but don't spend too long heating the pins as you can cause damage to the switch housing. After the PCB is firmly attached to a few switches, consider plugging the board in an testing the switches you've soldered. If they work, then continue on. Also, make sure if your board has PCB mounted stabilizers (the wires under longer keys) that you install them before you solder in the switches. Otherwise, you're going to have a bad time.
There's one last thing of which to be aware: your first custom board will not be your last. From the moment the solder cools and you start typing, you'll be wondering what else you can build. You might only need one keyboard at a time, but that doesn't mean you can't switch boards a few times a week… or day.