The trick is to use EVEN chunks. You want not too big, or too small - if too small, you will have lumps, if too big you will have gaps.
Thin the mane where it is uneven. Make sure it’s all one thickness and length (no big gaps in the middle). Clean, but not too conditioned. It’s easier to start with a slightly damp mane, but not too wet. Spray some quikbraid or hair spray along the crest before you really begin, Brush, then go.
I’m 6’ and always still require a step ladder to stand on to make absolutely sure that I am right on the crest. My thumbs sit as close to the skin as possible.
It takes a TON of practice. This past year I have gotten so good at it just through constant practice.
If you have problems with the mane not being long enough or holding, run yarn into the braid. You could even run a thread, cut and tie 1/3 down, add another yarn in, cut and tie 2/3 down, etc. You can also tie yarn in after you are done in areas that tend to loosen.
Honestly though - it takes technique, which takes practice, which takes messing it up a few times.
Attaching one of the braids that I did for a clinic last October. Loved how it turned out.
https://imgur.com/a/6Gt2BiN