"one piece grip" typically arrives in 3 cps. two grip panels and a "block" that is glued internally.
to install you only need to remove existing grips and roll pin. fit your new grips to frame and and mark the hole for the roll pin. drill out the grips for the roll pin and and epoxy the grips to the block, one side at a time. put a thin coat of vaseline on the frame so the epoxy won't stick to it. you can fine tune the fir after the glue sets.