All good advise above. Let me go a bit further.
If you have the correct Factory grips, there should be holes in the lower front edge corner on the inside of each panel, that fit over a protruding pin in the grip frame. That's the "locator" pin. Check that first.
Next, the spacer mentioned above.
I use the nylon ones vs the aluminum ones for a couple of reasons. One, while the grip frames SHOULD be 1/2" thick, I have often found the polishing process has them a few thousands thinner. This causes a grip panel to not quite seat against the gripframe. As such, I'm able to lightly sand off one side of the nylon easily to fit the guns. Next, I prefer nylon over the aluminum ones so that I don't get a dimpled ring in the back of the wood when I use them. If an aluminum one is used, and you do have to tighten it a bit, the aluminum ones will indent the wood just a little.
Just how I do things.