Here are some photos of a Giles 300 I built last year from Procraft plans, the original design was built up construction but foam is faster and easier. The photos are of the stabilizer however the same process was used for the wings, I got the digital camera after the wings were cut but not sheeted.
The foam block is first cut to the exact outline of the flying surface allowing for 1/2 balsa leading edge, the trailing edge is foam so the cores taper to almost nothing, I left about 1/32" of foam at the trailing edge to allow for melting as the wire pasts. 1/64" plywood strip about 3/4" wide is laminated under the balsa skin to provide a stiff trailing edge.
The second photo is showing the templates used to cut the access hole to cut the wing tube.