Go to a library and find a decent computer magazine, then scan through one year's issues. They will have at least one article on how to build a computer, perhaps even with a link to a corresponding video on the magazine's website. Over here, I'd recommend c't ( www.ct.de ), but that one's in German.
Anything else depends on your specific requirements. Reading that (or a few more) articles will give you an idea about requirements you didn't even know you had. And one personal tip: don't force it.
If something doesn't fit, check _why_ it doesn't fit. If I had done that instead of just pushing in the GPU, I wouldn't have needed a second motherboard :-/ A second tip: if you have to exchange the motherboard (yeah, the second one had a problem with one of the control lines for the second RAM bank...), double check that you have removed the cooler backplate from the motherboard before you bring it to the post office. You can't get individual backplates, so you'll have to buy a second complete cooler.
Anyone needs a Cryorig C1 with two sets of AMD mounting equipment?