basic spar construction

Can anyone suggest some home built construction sites where they show the
process of building a spar for a foam composite wing. I'm confused on how a
spar is built for a foam wing. It doesn't seem like the foam itself would
provide enough structure to provide a web. It would seem like you'd have
to cut the wing foil, cut out the location of the spar, build the spar, and
then reinstert that section back into the foam wing.

Thanks,
Dave