When first putting on the double sided grip tape make sure you extend it about a 1/4"-1/2 " beyond the end of the shaft, then fold the edges together and under the shaft and you now have the grip tape locked down so it can't slide forward ( and blocks any grip fluid entering the end of the shaft) when you slip on the grip in a fluid, forceful, steady push. I put some grip fluid inside the grip, blocking both holes, give it 3-4 shakes to lube the inside and then let the grip fluid pour out the bottom as I distribute it along the grip tape, then slide it on.
As others have advised I always put a protractor (magnetic) on the putter face to 90 deg and when I install the grip I use two (2) small bubble levels slightly up (1/3) and slightly down(1/3) from the top and bottom of the grip to level it off as well. Easy peezy every time!