I think I did something along the lines of attaching one wing, then using long nose
pliers, inserted the spring through the opposite hole in the fuselage, hooked it over the
hook in the wing, still holding the other end with the pliers, pulled it out of the fuse then
trapped it with a screw driver or similar. The spring is not that strong so it is not too difficult.
I then got the other wing, pulled the spring out a bit, hooked it over the hook in the wing, and
put the wing onto the fuse, did up the bolts, and so on. It's probably easiest to do before fitting
the tank and engine so you can look through the hole in the firewall and see what you're doing.

If you struggle to hook the spring on after fitting one wing, I suppose you could also attach the
spring to one wing, attach a piece of string to the other end of the spring, thread the string through the fuse,
pull the spring through the fuse as you put the wing on, then hook the other end onto the other wing,
then fit the wing etc. Again, the spring is not very strong so will pull out quite far.

Good luck!