I've been looking for a guide online for you, it seems there are non.
Anyways, it's a simple task if you are handy with the tools. You'll need a Hacksaw to chop the neck off, cut as close to the base of the skull/jaw as possible, and with a dremel and sanding bits, remove all unwanted plastic or resin. There is no steps to this, you'll have to mod this to fit your specific body type.
If you are modding a resin sculpt, you'll need to drill a hole for the neck peg, you can do this also with the dremel and a drilling bit.
This is the way I do it. Again, it sounds easy if you are handy with tools.