What kind of plane is it? There are a couple methods I use. The easiest is 3M 924 Transfer tape. Get the 3/4 inch, cover every bit of foam, peel the back and stick the balsa on, but get it right, once it touches, its stuck.

Next you have the old think down the yellow wood glue with water, paint it on the wood and the foam, let it dry, then iron the wood on like covering. Have done that many times and it works amazingly well.

Also, 3M 77 spray glue works great, but it has some really nasty stuff in it. Do it out doors and wear a charcoal filter resperator, not one of those cheap paper things.

All 3 work great and are very easy. I have used all many times and never had sheeting pull from a wing.