Most start at the head. With the P on the butt cap up most rackets are strung with the short side on the left side of the racket.
When you start stringing the crosses if you start on the wrong side it will not work. Also the string want to naturally pull into the top of the O port when you tension it. You must have a boomerange or some spacer to hold the string down in the bottom of the O port.
I would like to apologize to anyone I have not offended. Please be patient I will get to you shortly.