Practice...that's literally it.

use references

Arm are about the same length, biceps and triceps same size. Same goes for legs, but legs are about the size of the torso. Torso has a slight curve to it, the chest starts about above middle. Arms come down to mid thigh. Necks usually start from the ear, and go down curving from the chin, leading to the shoulders which spread out more than you'd think. I hope this helps, it's just the art tips my dad gives me ^^