Prepare both panels this way. Don't worry about the top of the wing that will fold over at this time, we will prepare it after the spar is glued to the bottom.

Mist the bottom of the wing panel with water. Be liberal. Also, mist the bottom of the spars. Lay a bead of poly glue on the spars (only one half) and carefully lay it flat on the first wing panel, keeping your main spar line lined up with the spar. Don't worry about the servo mounts in the pic, I'll get to that later.