I fit butt pads as a sideline to my work as a cabinetmaker. There are guys on the forum who have done a lot more of this work than I have but I'll tell you what I know.
A stationary disk sander is by far the best way of grinding the pad. An eight inch machine would work but a ten would be better. You can use a belt sander but the disk sander excels because the work is being pushed into a table as opposed to your holding it against the force of the moving belt. Bigger is better since a ten in. machine allows the entire pad always to be on the downside rotation of the disk.
To fit the pad attach it to the butt and scribe the outline with a sharp awl. Remove the pad and fill the scribe line with chalk.
The difficult part is maintaining the angle at the toe. The only practical way to do this is with a jig that can be set to maintain that angle. There are a number of them on the market. You can see a couple of different ones in the Brownell's catalog. I tried one of them and plan to go back to using the one I cobbled together with scrap plywood and hardware.
Once you have the jig set to the proper angle it's a matter of sanding to or just taking the chalk-filled line. Depending on the type of jig you may have to remove the pad or re-set the jig to grind the heel and sides.
I'm sure if you do a search on the board you'll find better and more detailed explanations of all this.
The jigs seem to cost $50-60. Not sure if you have a sander. From a practical point of view unless you have several pads that need to be fitted it may be cheaper to have a gunsmith do it.
If you decide to give it a go e-mail me if you have any questions or need any clarifications and I'll be happy to help.
SK
