It kind of comes to you naturally when you try it a couple of times. Besides Zarya, Soldier and Bastion (in Tank configuration) are also quite good at rocket jumping.
The order is:
Look down -> Jump ->(Immediately after the jump, but a bit of distance off the ground) Alternative fire.
It also works for Phara, but not only with her rockets (though they work just fine and give plenty of upwards lift). I'm a Phara main and hardly ever see this used:
Make sure the sky is clear, and there is nothing to stop you as you go really high really fast -> Look down -> and use both her abilities at the same exact time.
This boosts you really really high in the sky, I've been called hacker and glitcher once or twice because of this.