Put about half a shank's worth of 0.015 wire on a 2XL nymph hook — optional weight, but useful on a fly with no bead — dam the thread in front of and behind the wraps and run it back almost to the start of the bend.
Pull a small pinch of squirrel hair off a pine squirrel zonker strip and tie it in as a tail about a hook gap long, maybe a touch longer; wrap back over the butts to fill the step behind the weight, then snip the surplus so the body stays smooth.
Wax the thread well before you dub — natural squirrel is spiky and slippery and does not want to stay on bare thread.
Dub a loose noodle of squirrel hair dubbing about three inches long, kept deliberately untight so it stays spiky, and wrap the body forward. With a hundred percent squirrel, grab the waxed thread into a dubbing loop and spin it instead to hold the fur.
Brush the body out with velcro or a dubbing brush if you want it buggier.
Pull off twice as much thread as you need for the collar and wax it. Cut about an inch of squirrel fur close to the hide, hold it in your material hand, and form the dubbing loop with your spare fingers.
Lay the fur gently into the open loop and let the two legs close on it — this is the only fiddly part of the fly — trim the butt ends a little shorter than the tips, then spin the loop into a fur hackle.
Wrap the fur brush forward as a hackle, four or five turns, pulling the fibers back after each turn as you would a feather; tie it off and snip the loop out.
Sweep the collar back with the thread and build the head behind the eye — this is a nymph, so a fair-sized head is fine — trimming any stray fibers that would spoil it rather than trying to bury them.
Whip finish, add a drop of varnish, and rough the fly up with a brush if you want it scruffier still.