You must cast the rop while 'inside' the bubble that turns you into a skeleton. If you cast it from outside and then move inside (even if the rop is inside) it won't be active on your character. It's not difficult but can be a pita to get used to if you like standing in that spot where you can jump to reset stacks.
I've never had issues casting it from the outside. It's as the guy that posts the diagram said, when you try split it it bugs out, just stick one with the centre firmly inside and one with the centre firmly outside, and you should be covered.