I find it's useful to overlay drawings over the reference. I tried to line it up with the mouth/nose/eye on the left side and the eye/side-bump/top/back on the right. If you're trying to match (this) skull, it looks like you need to pull the jaw back a bit. However, different skulls will have different appearances. I think it looks cool the way you did it, though! You did an amazing job with the shading!