This one's actually pretty easy guys and gals. Of the options below, I have tried them all but usually use option #1. I'm not a real wacky worm fan, but have caught bass on them.

The neatest way is, of course, hook on a 4" plastic worm -- but then is it fly fishing? So this doesn't count as an option.

To be more fly fishing creative you have to address the issue of getting the fly to sink.
Option #1 is to use tube fly tubing and don't seal the ends. Hook a 4-6 inch piece crossways through the middle and fish away. Because tubing comes in a variety of colors and diameters, you can make a variety of wacky worms.
Option #2 is to cut three 1/4 inch wide strips of fun foam 4-6" long -- olive and orange and yellow. Glue them together (long ways) with super glue so you have a foam sandwich (like a chernobl on steroids). Taper the ends with scissors. This color combination is called wild fire tiger (hot bass color). Hook the WFT worm through the middle on a lead headed jig hook (otherwise it won't sink).
Option #3 is to buy wire stem chenille from Michael's or Hobby Lobby. Cut a piece and twist around the hook shank. If you use a barbed hook (shame on you) it will not come off. If you use a barbless hook (applause) you will have to put a layer of thread on the hook bend and then super glue the chenille to the thread.