Parting technique

I can braid a long rope braid, but can't do cornrows, french braids, etc. I know how in my head, but can't get my fingers to cooperate. Also my daughter is 2 and really doesn't want to sit still for me to practice.

Someone once told me that learning to part is half the battle, but I can find absolutely no articles on it!!! My MIL told me there are books, but some are better than others, and she couldn't think of any to recommend. BTW I'm not talking parting specifically to braids. I also don't know how to part for multiple pigtails or rope braids. Past pigtails, I'm pretty clueless!

Anyone know of any websites or books? Even just pictures and not instructions would be a big help. Thanks in advance!