Practice, and a lot of it.

Start with surf servers, that'll teach basic air strafing movements.

Next, find an MGE server that'll let you play as medic against soldiers and demos.

I learned how to jump as a soldier, and just transitioned those skills into surfing rockets as a medic. It's all timing and strafing.

It's essentially the same for every class though. I learned to surf rockets playing scout vs soldier in MGE.

If you are good at rocket jumping, the concept will come a lot easier.

spam spacebar and crouch

It doesn't start off easy. It's just like learning to rocket jump, if you don't know how to rocket jump, it'll be a lot harder to time your surfs. Also, doing rocket sync jumps in jump maps help, as you need to time the jumps with rockets.

the basic move is to correctly time your jump and duck with the incoming damage. add in an airstrafe (let go of W and hold A or D + move your mouse in the same direction at the correct speed), and you can control your path through the air. The trick is the timing -- if you press jump and duck at the time that you receive the damage, you'll go the farthest. if you go a bit earlier you'll get less distance or height, depending on the situation. if you miss the timing window then you won't really go anywhere (i.e. too early or too late). so it helps if you can predict what your opponent is going to do.

you can surf any damage including rockets, stickies, pipes, and hitscan weapons.

alias +duckjump "+jump;+duck"
alias -duckjump "-duck;-jump"
bind SPACE "+duckjump"

Really helps me a lot as Medic. Also recommended by Mirelin.

i don't recommend this, as servers like surf iT have heavily increased aircontrol over the default game

play mge and surf damage. you'll eventually do the same in games. I don't ever play medic and i somehow surf damage. or practice syncs as soldier since it helps for timing

Jump as the rocket is exploding. You are basically just rocket jumping with their rocket.

It's all about timing tho. Jump too early or too late and you won't get a good surf.

time when rockets are going to hit ur feet

It's really similar to hitting synced rockets in trickjumping. I guess the only other tip I'd have is be aware of your ping, the higher the ping the further away the rocket will appear when you have to jump.

press z or r twice

wait that's barrel rolling

It's like timing a reflect as pyro. You have to know when a soldier wants to shoot a rocket at you and react quickly once he does. 90% of rocket surfing is the timing. If you can, go with a friend on an empty server and have him shoot rockets at your feet until you get used to how quickly you have to crouch jump once you see the rocket.

play spire

Something I don't think people have talked about yet (probably because it seems kinda obvious) is that the placement of the rocket is very important. If the rocket explodes directly below you, you will fly straight up, which is bad. If you jump over it and it explodes behind you, it will propel you into your enemies, which is usually not the desired effect when playing Medic. You want the rocket to explode right next to you, but in-between you and your enemies, so you are propelled away from them to safety.

This usually works but occasionally the soldier will wait half a second for you to start touching the ground so you barely go anywhere. It usually happens to me when a soldier is chasing and there's nothing else for him to worry about except killing me.

play soldier mge and surf rockets, it'll become natural

spam space and pretend that your surf was intentional

play medic at pubs and just work on it. Its not too hard after a while. dont expect invite level surfing after a week

Can't... let... go... of W... Pyro... past... back to haunt me...

first you jump when a rocket hits your feet

then you're done

don't do this unless you want to fuck up

Play a bit of pub as medic, just to practice the timing. Also, strafe after you surf the rocket/sticky, very important. ;)

Situational awareness is key. Make sure you're looking at the threat you're surfing away from. Slin's description of the strafe mechanics is perfect, but it's a lot easier to time your reaction to a threat if you can see it or at least know where it's coming from. You'll have more success backstrafe surfing off a rocket you can see than blindly +fwding and spamming crouch+jump while running away from a crazed roamer.

For me, the crouch jump script makes it easier to surf. My PTT key is LSHIFT, so I'm free to comm (scream hysterically) about the nature of the threat or where I'm landing, without having to hold LCTRL with my pinky.

Happy surfing.

Play MGE. It's all anyone seems to do.

what the fuck

Play soldier more often and learn how to rocket jump. I've never really had a problem surfing rockets when I play med simply because I'm so used to jumping off of my own rockets all the time.

tr_newbots against a soldier works nicely with the unlimited health option for local practice.