To build a tapered thread body on a fly like a Zebra Midge, get your tying thread started on the hook shank and snip the tag off close. Lay the wire for the rib against the hook and begin securing it with wraps of tying thread. Continue making touching wraps well down into the hook bend, then wrap back up the shank to just behind the bead. Now for the taper. Make nearly touching wraps of thread over 1/3 the length of the body. Then wrap back up to the bead. Next go 2/3s of the way down the shank and back up. Finally, take wraps over the full length of the body.
Before starting back up, give your thread a good counterclockwise spin to uncord and flatten it. Doing so results in thinner, flatter wraps which will help to smooth out any lumps or bumps as you wrap forward. Every few wraps, give your bobbin another counterclockwise spin to keep the thread nice and flat.
A smooth, tapered body created in this manner provides an even platform on which to wrap the wire rib. This makes it much easier to produce good looking segmentation and helps with consistency between flies.