O hai thar!
On the old CYMA I had, you needed to remove the gearbox from the "donor", this would give you access to the plate inside that holds most of the stock in place. There's also a grub screw or something similar on the outside of the "hub" for the stock, which you remove and the whole assembly should come away. Then it's just a reverse-engineering process; it'll go on the recipient gun in the reverse order you got it off.
I'm sorry I can't be more help, but it should be fairly simple. You basically look for the screw (could be a teeny grub screw IIRC) and any plate on the inside that holds the thing on (could be a plate or a screwed on cylinder, I don't remember), undo it all and it's a straight, clean swap.