Spin clumps of golden brown deer hair on the shank: take the thread all the way around the hair to gather it, and on the second turn pull down and let go so the hair spins around the hook. Five or six clumps fills a hook this size.
Pack each clump tight before adding the next. Deer hair is what gives this dragon its buoyancy. The 'double float' name comes from having both hollow hair and foam holding the fly up.
Shape the spun body with a razor blade first, taking off the bulk in long slices, then refine with scissors into the flat, broad-shouldered wedge of a real dragonfly nymph.
Reattach 100 denier Nano Silk at the front and tie in a bunch of pheasant tail fibres for the legs. A strong thin thread is needed here because clipped deer hair fights back.
Tie in a strip of 2mm craft foam, poke a hole through it with your bodkin, slip it over the hook eye and tie it off. The hole locks the foam in place and keeps the wing case dead straight over the body.
Trim the excess, whip finish, and dot two eyes with a Sharpie. Fish it deep over a drop-off on a fast sinking line and creep it back up the shelf. The floating body keeps it riding above the bottom instead of snagging.