If you value your fingers put some gloves on first or it'll slice them right up

Coil the spring into the plastic housing and then coil the cord, by removing the centre hex part you'll then be able to seat the centre part of the spring properly. Pull the cord slightly to build up some tension in the spring, and then without allowing the spring to pop out you want to wind a revolution or two of cord around the plastic housing (you'll have to get it between the plastic housing and the outer casing without letting the inner spin under the tension of the spring). Done a few times you'll build up enough tension to retract the cord into the housing fully.

Then just re-insert the centre part and bolt the pullstart back on. It's fairly easy to do once you've got your head around it...