I'm gonna go ahead and guess internet/YouTube
And to be honest the basics are really easy, it only starts to get a bit finnicky once you get into the details.
The easy level is buying a board, buying switches that are supported by it, buying a set of stabilizers and just assembling all that. Oh, and keycaps.
Advanced level is foam/tape mods, lubing and filming (inserting films) the switches, that kind of stuff.
The 'hobyist'/enthusiast level is combining different switch parts to make a custom switch, experimenting with internal padding/gaskets, and gennerally fiddling with it and seeing what happens.