This is how I do it.

I have a square piece of aluminum extrusion with sandpaper spray glued to it. I have another piece made from 1 x 4 that has sandpaper glued to it that I used to grip the balsa sheet and slide against the extrusion. That gives me a perfectly straight and square edge.

Also, match the sheets. Be sure both edges of every sheet mate against both edges of every other sheet. The reason being that often balsa sheets taper across their width (mill defect).

Tape the sheets together on one side and flip it over. Use Ambroid or Sigment to glue them together - not CA. CA is too hard and will never sand down right unless you're using too heavy of a wood. Always use the lightest wood you can get for a completely sheeted wing.

Once the glue is in place, press the seams flush with the masking tape side down. Get a paper towel and acetone and wipe up excess glue. It will make sanding much easier later. When the acetone evaporates (about 20 seconds later), put masking tape on the other side.

By the way, you only need strips of masking tape going across the seam about every 3" to 4" inches. Don't run a strip down the whole seam.

Give it a good 8 hours minimum to dry and then sand, sand, sand, sand and sand some more with good sanding blocks.