Walt's Worm is about as plain as a fly gets: tan hare's-ear-type dubbing wrapped on a hook, no tail, no wing, no flash in the original. The fat, buggy tan body reads as a caddis larva to a trout, which is why it earns its keep as a searching nymph through much of the season. Walt Young tied it for Pennsylvania limestone streams, and it has been a Northeast and Midwest staple ever since.
Sizes 10-16, tan only - this is not a fly you fish in a rainbow of colors. A tungsten bead version, sometimes called the Sexy Walt's, gets it down faster in deeper water. The plain beadless version rides higher in the column, which is what you want for shallow runs and spooky fish.
Fish it dead-drift near the bottom, the way caddis larvae actually move, which is to say not much. It works best in clear to lightly stained water at normal flows, with trout most active in the 44-62F band. Run it as the point fly under an indicator, or as the dropper below a heavier nymph in a two-fly rig.