How Do I seal cement stepping stones for painting on them?

You can use a concrete sealer, then, after you paint, use something like Rustoleum clear paint

You can seal them AFTER they are painted. Ask local DIY Center for what you would seal them with.

I paint a lot of cement stepping stones. Best ones I've done were sealed 1st with 1 coat of Krylon outdoor matte sealer. Then painted with outdoor craft paint. Then covered with 3-4 coats of the same sealer. Sometimes krylon glossy - depends on the look I am shooting for. This is an example of one I did 15 years ago that was matte. It's needing to be cleaned & redone - not one we step on, so it's lasted a long time.