Gary LaFontaine’s Sparkle Pupa is one of the most effective flies ever, period. But people shy away from tying it because its expanded sheath or bubble can be a hassle to get right. Here’s an easy method.
Start your thread at the mid point on the hook shank. Snip free a 2” length of Antron yarn. Lay the yarn’s midpoint on top of the hook shank and secure it with wraps of tying thread. Sweep the Antron rearward and bind it down to the start of the hook bend. Work the Antron down on both sides of the hook. Then take thread wraps to in front of the hook point.
Create a short, thin dubbing noodle on your tying thread and take wraps with it to form the underbody of the fly. This should extend no more than 2/3rd of the way up the hook shank.
Make open spiral thread wraps up the shank to behind the eye, like 3 or 4 max. Pull all the Antron forward and take 3 or 4 thread wraps to anchor it. Gently pull rearward on the butt ends of the Antron to automagically inflate the bubble. Taking a few wraps around just the hook shank, followed by a few more around the Antron and the shank, will lock everything in place.