Start from the corner or one end. That way as you work your way round you can adjust the pleat size to a) ensure you have enough bag for the detail required, and b) ensure you don't get to the end and have loads of bag over the end or need really big pleats at one end to use it up.
Beyond that there is no set right or wrong as long as you have enough bagging material to get properly into all the recesses and depth of the mould.