Attached is step-by-step instructions for the San Juan worm. One important point is that Ultra chenille is used, not regular chenille like used on wooly buggers. Tie it like in the attached instructions, and also try using a curved scud/pupa type hook with a beadhead. Sometimes I also tie lead wire on top of the hook shank to sink the fly deep. The fly with the scud hook will flip upside down (hook point up) and will bounce on the bottom with fewer snags. Do the same thing with chartreuse chenille and you will have a modified green weenie. The green version works well to represent green free-living caddis larva. I hate using these junk flies, but they sure work.
http://www.charliesflyboxinc.com/flybox/details.cfm?parentID=106