Mount a needle eye-first in the vise. From a 1/8" thick sheet of foam, cut a strip 1/4" wide and 3" long. Fold the strip in half, and push the foam onto the needle at the center of the fold. Mount the tying thread on the needle 1/4" in front of the foam.
 

 
Fold the ends of the foam forward, sandwiching them around the sides of the needle. Take four firm wraps of thread around the foam to form the first body segment. Then fold the foam strips back and advance the thread forward a distance equal to the width of the first body segment. Fold the foam forward again, and take four more thread wraps to form the second body segment.
Continue forming segments until you reach the desired body length. Secure the thread at the last body segment with a couple of half-hitches; then clip it. To remove the body extension, pinch your fingertips against the needle behind the body and push it off.
To mount the body extension on the hook, attach the thread at the rear of the hook. Sandwich the foam tags around the sides of the hook shank. Take four firm thread wraps to secure the foam to the shank and to form an additional body segment. Form more body segments if desired, then clip and bind down the foam tags.