I sew the webbing directly to the fabric of the hip belt (or shoulder straps). I sew the webbing to one of the fabric pieces with bar tacks and a perimeter stitching. I then sew the two fabric pieces together with the right sides in and then roll the fabric over the foam bringing the right sides to the outside. Then, I will stitch through the fabric and foam in several places to bind the assembly into a unit. This last step is important, I think, because otherwise the fabric will shift on the foam and the webbing can wind up riding you where you least want it. BTW, I'm making all of this sound easier than it is.
May I walk in beauty.