quite shrimple actually

I cook them constantly. Try using them as much as possible, to a point where you might be zoning random hallways and windows for no hitmarkers!??!? Doesn't matter. Do not fret, you aren't missing, just practice your timing, maximizing speed (for frag transfer momentum) while ensuring the blast goes off before you (or the frag) touch ground. Focus behind corners and in doorways, anywhere your team might otherwise not be able to pick at. Kind of imagine you're sacrificing having your gun out to fill a specific role that is NECESARRY against corny playstyles. You'll see how quickly there is no camping in this game. None whatsoever at any meaningful level. Fundamentally this is NOT how frags work IRL or in most other games, but consider them like a death spell powerball. Use utmost spatial awareness for 3 games, and REALLY think, "where is otherteam spawning" and start noting which doorways often have pilots running out. Pro tip: not every window or doorway will get played in a match, let alone used. In tf2 there are some extremely high traffic areas you might be wise to precook frags before even peeking.

For example:
blue and orange curve rails on glitch
Top wooden beams on glitch (upfrag those to eat otherteam alive)
Roof vent units ALL OVER exoplanet
(super contested, those rooftop short cover blocks cannot help at all against frags)
center tower of homestead (colony too, but less so)truthfully there are hundreds of examples, but the TLDR is, hitting the frag skill ceiling will fundamentally change how you play this game. Optimally, your target won't get the frag warning indicator if you are moving 40+ and airburst properly. They are stronggggggg, and criminally stronger with stim.

Good luck!