When wrapping floss bodies on flies, particularly when using multi-strand floss, here are a couple of tricks for keeping everything neat and even.
The first is to have very clean and smooth fingertips so as not to fray and/or discolor the delicate floss. It’s also a good idea to leave a fair bit of distance between your fingertips and the hook while you wrap, so you’re effectively wrapping just clean floss.
When making a turn to double-layer the floss, the strands often want to separate with one or two wanting to run up the shank before the rest and causing problems. To solve this, try wrapping the floss behind your tying thread so, with each wrap, the floss has to push the thread up, which then sandwiches the strands together. At the turn, still wrap behind your tying thread, but up the shank instead. Doing this usually results in a much smoother, even body on the fly.
These methods work for both multi-stranded and even single-strand floss where the fibers within the single strand want to separate.