Get them as close as you can. You may have to sand a little. but don't sand unless you have to. It should go in the pocket fairly snug. Make sure you get them in the right direction. Don't put them in upside down. Do one side at a time. then when you are ready to join the other wing half put the epoxy on the joiner and on the root ribs. Slide together, get them snug, put some tape across the seam to hold them while the epoxy sets. This is the time that you want to take the alcohol and clean off any epoxy that squeezed out. You may have to do the alcohol thing a couple of times as it is starting to harden. make sure that the wing halves don;'t move while you are waiting for the epoxy to harden.

side bar, I put a couple of small clamps on the wing joiner until the epoxy sets. You may also want to use the alcohol to clean off the wing joiner so it will be able to slide in the pocket.