Mechanical Keyboards

Ok, so you said put together your own keyboard, that involves soldering. However it's very likely you don't need to.

Finding which switches you like is a good first step, as a general rule I think there's only 3 types of switches you should worry about, let's call them Red, Brown and Blue since that's the colors Cherry MX uses for them so they're sort of the standard. Red are fully linear, i.e. they feel the same from start to bottom. Browns have a small bump midway through (when the switch activates). Blues are like Browns but they also make a click sound. Only choose colicky switches if you have a room for yourself, they can be VERY annoying to other people, be considerate. That being said it's personal preference, I personally like Browns although I have used Reds and honestly I don't feel that much of a difference.

Next important is figuring out the size you want, do you plan on moving it a lot? If so a smaller form size might be better.

Then there are some ergonomics, personally I love Split ortholinear keyboards, you can buy premade ones but for me it was cheaper to build one for myself, but I'm okay with soldering. That being said if you're going to solder, I STRONGLY recommend you get a nice modern USB-C pen style soldering iron, I bought a cheap one from Amazon and it was very difficult to use, didn't heated up properly and had a very large tip (the small one never got hot enough to melt the solder).