When i try to inset faces as he does at 8:00 for some reason a extra edge is created instead of the inset slipping to the mirror.
You'll need to uncheck "Boundary" in the menu on the bottom left (or press B after I to inset).
You're using a mirror modifier, which means half of the model is just a mirror image and not real. Because of that you have an open face in the middle, meaning a face with an edge that is not connected to anything on one side. The boundary setting of the inset tool lets you decide whether to inset from that open side or not.
