Well, before you do anything, you'll have to take precautions. Rabbits, for the most part, really dislike being wet, and they're slow to dry which can cause other issues down the line. So you'll need to be prepared for you or someone you trust to hold on firmly to your bunny and avoid struggling.
I like to take a soft bristle toothbrush or a washcloth with lukewarm (not hot, and certainly not cold) water, and gently wipe or scrub my Quinn's feet. Soap is generally not necessary and can be harmful.
As rabbits are self cleaning, like cats, you shouldn't need to do much more than that. Make sure their feet are as dry as possible with a clean towel before letting them down. They'll most likely do whatever is left themselves.
If they continue to get REALLY dirty, change their litter boxes/pans more often. It's a sign that they're digging in their litter.