i did the same a long while back.. the way i did it was.. take the cord off.. wrap the spring around the central part of the pullstart tight-ish then put the other end of the spring into its position on the outer part of the pullstart as it would be if it was fully together.. so you have the spring where it should be but you have both halves of the pullstart slightly apart.. then spin the inner part to tighten the spring.. as tight as you can, as you turn it tighter the force of the spring will pull the 2 halves of the pullstart together

when its nice and tight and the 2 halves are together re-thread the string through its hole in the middle bit of the pullstart.. put the whole thing back on its backplate as soon as you let go of the middle of the pullstart to get it on its backplate hold it tight and the whole thing will unwind.. pulling the cord back into it.. if youv done it right.. that should be the perfect tension for the cord.. and ur rolling..

it takes a bit of skill to do it without either getting cut.. or letting go by accident and getting a spring in the face or something lol

hope that all made sence :P