Lot's of ways to skin this cat.

My preference is to build the webs, sides, and back as separate assemblies
and then put them all together.

I've posted tow examples to ABPW

The first examble is done with a a stopped dado (1/8" deep) cut into the
side assemblies. I pocket holed from the underside.

In the second example I build the web with tennons sticking into the fron
legs. The rear of the we is supported by a dado cut in the intermediate
stiles of the rear panel assembly. It coulsdhave been done with mortises in
the rear, but it was a really wide piece and I wanted support in the middle.

Note that the top web is a special case. It's set in with a dovetail from
the top.

-Steve