If you are lucky, things will fit reasonably well from the start. The scenario that will give you headaches is if when fitting one side into the boss pin, the grips are a no go at the top. You can try and lay the grip onto the boss pin and eyeball the top to see where it might need to be taken down, but this will be a pain. If you drift out the pin from the frame, then you can slide the grips around easily to check fit at the top, then while holding them into position, flip the gun over, shine a light through the boss pin hole and you can see if the hole need "tweaking". You really want to avoid trying to take material off the top until you understand the overall fit. If you bugger the angle up top (it is not 90 degrees) you will get into trouble quick.
And, you will want to avoid hammering the boss pin out if you can avoid it. depending on how tight he fit is, you can slightly peen the head, then it would need to be dressed down to fit the grips again. If you have a big enough press/vise, that would be the preferred method of drifting it out. The pin is splined, and it does not take much in and out to make it fit loose. Hence my previous note about picking up a spare from FA's.
And in general, you will need sandpaper to remove excess grip material ( I know duh). When I fiddled with my first set of rosewood from FA, I used 50 through about 600, then switched to the various grits of steeel wool, and finished with a buffing wheel. Not hard to get a mirror finish on any sanding. Make sure if you do any heavy sanding, wear a mask, and do it outside. That rosewood dust is seriously bad news on the ol' nostrils and lungs.
I have actually been having fun making grips for my FA. My latest is a really neat piece of bastogne walnut that when I picked it up, I got enough to do a set for a Bearcat and a nice damascus blade that my wife bought me. It makes for a cool set, 83, Bearcat and knife all in matching wood.
Heck I was just happy to get those rubber grips off my FA in the first place.
Craig