Okay, you got 1 skin on. Some guys will do both skins at once--but I've screwed it up before, so I just do 1 at a time now. I'm not the best builder either.

Just pull the wing panel out of the cores and do the same thing on the other side.

When you have both sides sheeted, now you can sand off the excess on the root, tip and leading edge. Stand the wing up on the leading edge and sand it back and forth across a long piece of sand paper. Sand SLOW. Sand EVENLY. Don't get in a hurry.

If you do not want to stand the wing up on the leading edge to sand it--it will be fine to use a sanding block. But just remember to use a very long sanding block. At least 12" (30cm). If you get a sanding block much lionger than 18" (45cm) it will be cumbersome and hard to handle well.

SMALL SANDING BLOCKS ARE USELESS. ALL THEY DO IS SAND RIPPLES AND WAVES INTO BALSA. NO SMALL SANDING BLOCKS

Use a large sanding block to sand the excess off the root and tip.

Then glue on the leading edge balsa block. Shape the leading edge balsa with a razor plane or sanding block with 60 grit paper. When you get close--drop down to 80 grit and then 120 grit.

How you shape the leading edge is up to you. Some people try and shape them uniformly across the leading edge and gradually taper the curve to match the thickness of the wing as it tapers down and gets thinner near the tip. I prefer a very blunt and fat root section and an almost razor sharp tip. This helps snaps and stall.