Spray the handlebar and inside the grip with your hairspray or lubricant/adhesive.

Slide the grips all the way on until there's no more room for the grips to slide on.

I use hairspray because it is both a lubricant and an adhesive. Other combos can be WD-40 and super glue.

If your handlebar is dirty or you want to clean it, use warm soap and water to clean it with a wash cloth.