Polyurethane glue is my choice!

First true the edges of the balsa sheeting and then tape the sheets together (flat) using masking tape. Then with the tape side down spread a very thin coat of Polyurethane glue, use a old credit card to spread it as thin as possible. Place wing core back in the foam that they came in (protect from gluing together with wax paper) dampen the wing core with a very light misting of water and put the skin on the wing cover with the remaining foam and weight it down.

Use a lot of weight, cover the foam with a flat board and the put on as mush weight as you can (sand bags, iron weight used for lifting can be found at garage sales, etc.) or if you know someone that can vacuum bag that is better.

The moisture helps cure the glue and you will find that Polyurethane expands (like a foam) and will not only glue the shins to the core but also glue the seperate pieces of balsa together. Repeat for the other side and wing.

After a few wings you may be able to do both sides at the same time but do not try it on your first attempt.

Some modelers like to use Epoxy using the same method.