I feel like I can provide some assistance!
My hair is quite short now, but it was about waist-length up until I was ~13 and I wore braids a lot. My mom used to do them for me, but eventually I figured out how to do it myself (though not as neatly). Like like someone up there said, it's really just muscle-memory - I've tried explaining to other people how to do it and I get confused in the process. I can do my own and other people's pretty easily... A basic braid like the one Reynir's sporting is not that difficult, either - it's only 3 strands after all. I think as long as you divine the hair into an odd number of sections, you can make it as wide as you like (provided you have enough hair to work with, that is)