Hi!
Here is my tip from doing many airplanes!

The best way of starting is to tape the balsa sheets together! Ca is not that good and neither is using Aliphatic glue! Why? Because you get a hard rim of glue that must be sanded down once the planking or wing sheeting is done, which is rather hard to do. Using just tape makes the balsa joint invisible.

Second! What glue to use???
The best way of planking a foam wing is to use either 24 hour epoxy resin (not the hobby epoxy glue variety you get in the hobby shops ...but professional 24 hour laminating epoxy...or to use Poly-urethan glue (Gorilla glue to you Americans).

When using these two types you must have the scrap foam material as a bottom and top layers in which the wing core is secured together with the balsa sheets.
Heavy iron clamps and thick board material (60mm-80mm) is then used to really clamp everything together.

I do not recommend Dave Brown Southern Sorgum contact glue or any aliphatic or white glue as these glues cannot be used in the above manner!
When using those you just let them dry before you roll em onto the foam wing...which is not a good method to use if you want a straight wing! Even if it could be done...