If we are talking standard grips...make sure the cross arm lock bar deal is out of the way and insert the grip, then I use a flat head screwdriver, but I cover the end with tape to not scratch the frame if i slip on the inside and move the arm so the bar locks it into the frame. I put the one side on first and move the bar from the other side of the frame grip hole opening, then I put the other grip on and turn the gun upside down and there is that bar with the naotch and I move it from one side to the other to make sure its locked in. Sometimes those lock bars for a lack of a better term can be jammed wirh cosmoline and hard to move, so I clean 'em up and make sure I can spin the bars without much effort. Also sometimes it akes some effort to get the grip set in place as the grips can be warped a tiny bit and this complicates the job.
I'm a terrible explainer and the best advice I can give you is youtube video on disassembley. Give one a watch and you'll get it quick.