Well, you need to know how to operate a 3D printer, and unless you happen to find files for the grips you want to make you need to know how to use some sort of design software to design your own.
For revolver grips you will also likely want to do some kind of post processing (filling, sanding, etc) as I don't think you'd be able to print in an orientation that'd give you a nice surface finish... I could be wrong about that though.