I would put the new grips on and scribe a line around them to mark any excess grip material that hangs over the frame.
Next, you have to remove the excess somehow.
Without power tools like a fine belt sander and buffing equipment, your safest method will be with fine-cut files, and progressively finer sandpaper to bring up the final polish.
Tape/mask the frame with black vinyl electrical tape to protect the bluing, and do the final fitting with the file down to the frame.
Now take them back off the gun and do the final smoothing with 240, 320, 400 grit sandpaper to remove the rest of the excess and the file marks.
Finally, if they are plastic grips, use Crocus Cloth followed by auto rubbing compound for the final polish.
If they are wood, skip the crocus cloth and rubbing compound.
Use Birchwood-Casy Tru-Oil to finish wood grips.
I do not recommend using a Dremel tool sander as some will probably recommend, as it is impossible to keep from making scalloped edges all over the grips. Or even worse, nicking into the metal grip by accident.
rc