Depending on the mod.
Usually in aikido servers people would place thier arm an the side of the head, and contract all thier arm joints. This would act like cutter and pop the head out.
in other servers that have a less dm threshold usually a solid smack with your arm would do the trick.
There are no easy or hard ways to decap. Just practice and maybe watch somebody else's replays with shift+p to see how they do it or get some inspiration.
There's no easy way to decap without a given scenario.
If you're asking for a move that'll decap someone everytime for any move, you won't find it.
Practice and find out yourself.