Specific things like this are rarely explained because understanding the broader topic of drawing characters in perspective is all you need, and there are many resources on that.
Loomis's Figure Drawing book has a fairly simple, but detailed explanation of how it works, in addition to a lot of other useful things.
Teaching and learning specific topics where broader ones would do will just set an artist back and waste their time in the long run. When learning anything, don't think "this is what I need now, so this is all I will learn", think for the longer term.