How do I clean drywall mud and primer off of my stairs?

The mud should wash off with just water repeat twice one to take it away the second to remove the film it leaves. as for the primer if its water based it should remove fairly eisily if your stairs are textured then id try a toothbrush or something like that with a mild dish soap or detergent. If no texture and the stairs are smooth finished it should just pick right off just take your finger nail and scratch the primer up.