when you pin it onto the leaders, you have to make sure that you have the top even, start in the middle and then go out, Then as you roll, you have to make sure that you adjust as you go, tugging here and there making sure that your edges roll even like a bold of fabric on a roll. It is easier to do with an extra pair of hands or two. The as it unrolls you should have the side clips to help keep it taunt.
I hope this helps