Get the boots that reduce rocket jump damage and then rocket jump everywhere to get a feel for it. Then take the boots off and rocket jump everywhere until you learn to do it without killing yourself.

All rockets require various degrees of shooting where the enemy will be, rather than where they are, but it also helps to shoot where they might be - drop a rocket just past a corner or through a doorway just in case someone's approaching from the other side. Worst case scenario you can be confident nobody's right on the other side.

You can surprise people around corners with rockets, but if they can see you firing they're probably gonna dodge it past medium range. Get closer so they cant.

Just like you can rocket jump off the blasts, you can knock people around with them. Throw them at the walls behind snipers to knock them off their perches. Knock the scout into the chasm. Separate the medic and heavy.