Pappy,
attach the grips in the rough on the frame, making sure they are nocked up and square into the angle formed by the frame and the upper grip frame, then take a thin pencil and mark along the underside of each panel where they may stick out proud from the frame, remove and file down to the mark on both holding the file right angles to the inner face.
This roughing to shape will give you the outline. Wrap the grip frame with blue painter's tape, or similar, to protect against slips and nicks, re-install the grips and with a finer file round the edges working the file from the edge up and into towards the body of the grip panels to shape. Test frequently for feel in your hand. Finish with finer grades of garnet or sandpaper until smooth. This is where an emery board will make it easy.
When sanded to your satisfaction, remove and rub a wax candle around on the surface to give some tooth, remove the painter's tape for the grip frame and re-install your beautiful new grips.
Most pards fit grips themselves just to say they did it. It is not difficult, just tedious. Put a Western DVD in and view while fitting for inspiration, - keeps your mind occupied while your hands are busy. It will be a night's pleasant diversion.
And wear a mask agains dust kicked up from the resin used for your brand of faux grips selected, just in case you're sensitive to the chemicals.