Pretty sure you don't need the arm warmers to get double-unders, but then, you never know. I suggest skipping the fast singles. Once you've got consistent single-unders, go right to working on the big bounce with a single, then big bounce with a double. Also, ideally, your double-under body position is virtually the same as it is in triple-extension (top of the second pull of the clean and the snatch): ankles extended, butt tight, abs locked down. May the rope never trip your toes!