You need to give her a compelling reason to not jump off of the steps. She isn't going to be nearly as interested in listening to you tell her to slow down as she is going to be to get out in the yard and smell things. You have to be the bigger incentive.
So I'd get a high value treat and put her on a leash. I'd get her to the last couple of steps and stop her. Get her to look at you and give her the treat. You can repeat this on the way back up the steps as well, still stopping on the bottom two.
By doing this, you're creating a really nice association with slowing down or stopping on the bottom couple of steps. Then when she goes down them on her own, the thought of the treat that going to that step associates in her mind might cause her to slow enough no to jump. It'll probably take multiple repetitions.
It might also help to play a little before you go outside to knock that edge off her energy. Good luck.