Mount a 3X-long streamer hook and take red thread back to the bend. Tie in a small tuft of yellow calf tail about a hook-gap long with a pinch wrap, then a couple of wraps behind it.
Leave the calf-tail butts in place to bulk the underbody; secure them with a few loose wraps, then tight wraps forward to build a thick, even underbody, and trim the front butts neat.
Tie in tan chenille at the rear. If it is finer than you'd like, lay it back and forth in a couple of open layers to thicken the underbody, then wrap it forward in touching turns almost to the eye and tie off.
Build a red thread head over the chenille tie-off to cover the wraps, then whip finish.