In all honesty, I'm a "hands-on" type & help them undress if needed. But first they need to soften up the old shed, & a 15 minute soak often isn't enough.
You might try using a container w/lid, with enough room for both the snake and quite a bit of either: soaked & very damp sphagnum moss, or a small wet
towel. It might take an hour or several in there, but they'll probably be able to remove the old skin on their own at that point. If they don't, that's when I
help out. This container should have air holes, of course; I usually use a "critter cottage" for this (they have well ventilated snap-on lids) & just leave it in
their cage so they aren't getting cold...leave the container near but not in/over the heated end of cage, so they're at average cage temperatures.
With any luck, you'll have very little or no shed to help them with after this, but check the eye caps, chin, neck & tail tip especially.