Modeling trees can be quite a difficult task, and while there's
software out there to help you do it faster like speed tree, sometimes
the tree you're making is so unique or needs to be so detailed that
there's really no good automated way to do it, you just have to model
by hand! In these cases, here's a few tips I've found helpful to not
only speed up the process, but produce trees with real character, so
they don't look malformed or too simple.
http://www.neilblevins.com/cg_education/tree_modeling/tree_modeling.htm