How many attempts have you had? Are they all similar? If they are then you are doing something right.
Once you have the clingfilm and tape done, I think finding the hairline (4 fingers above your eyebrows) the next step. I mark this with a dot, then continue to dot at intervals around before joining up. I pay attention to the shape of the hair line. Definitely need 2 mirrors to make sure it covers everything at top/back.
It doesnt have to be mm precise, but as long as it covers the whole area and you have a good hair line it should be fine. Even if its a little off, it can be positions to best effect.
If its your first one often the stylist will prepare your area to fit the piece anyhow.