I have not tried this style with a foam post but it should work the same. Here it goes 1)tie in the tail or shuck and abdomen 2) strip a bunch of cdc fibers off the stem 3)lay them along side of your post material (I use poly yarn) tie the post and cdc fibers on at the same time in the center of the bunch do not trim them yet 4) pull all the fibers and post material up dub the thorax 5) pull all of the cdc fibers below the shank and trim them to the width of the hook gap they should splay out like the fly in the photo 6) trim the post to length. This method is shown in the book Tying Small Flies by Ed Engle and also in Mayflies Top to Bottom by Shane Stalcup.
Turk