First, remove the two screws on either side of the hammer, and the screw at the front of the butt. Remove the rear grip strap and the one piece grip, which will probably come along with the backstrap.
Now unscrew the mainspring screw and remove the mainspring (hammer spring). You can either remove the front strap and trigger guard or leave them in place. Try your new grips on the front strap. Trim them to fit. Then install the backstrap and trim the grips to fit. To prevent marring the straps, use a couple of layers of masking tape when fitting the grips. When the grips fit, remove the grips from the front strap, replace the mainspring and mainspring screw. Replace the front strap/trigger guard if you have removed that part before replacing the front strap. Replace the grips and rear strap.
One point. Try the "new" grips first before whittling on them; if they are takeoffs, they may already be too small and can't be fitted to your gun unless you want to remove metal, something I don't recommend.
(Note that the original Colt one piece grips were one piece; many modern "one piece" grips are actually two piece with a third piece filler block glued between them.)
Jim
