Step 1: Scrub it under the sink with some Bar Keeper's Friend scouring powder and a brush. Scrub, scrub scrub. Rinse off well. Repeat if needed. No need to let it dry at this point.
Step 2: Get some sand (clean while builders sand or the kind used for kid's sandboxes, not the reddish stuff you put on your lawn) or coarse 60/90 grit silicon carbide lapidary grit, and a flat surface like a piece of tile, glass, or my personal favorite, a flat area of sidewalk. Using some water with a just a little liquid soap in it, lap the stone to refresh the surface.
To lap the stone, you put your loose abrasive on the flat surface, wet the stone in your bucket of lightly soapy water, and while holding the stone flat on top of your layer of grit, you work it side to side, back and forth, in figure 8s, circles, W's, X's, M's, alternating direction so that you are moving it every which way, trying to avoid putting too much pressure in any one direction so as not to slope towards the edges.
So ideally at this point, you are on your hands and knees by your carport scrubbing the sidewalk with a sharpening stone on a layer of wet sand. You will eventually grind the sand into powder but don't let that happen - add more grit if it wears down, you don't want to glaze and polish the stone, the idea is to flatten it while getting it to a fresh layer of abrasive. You don't need much sand, just a sprinking of it, not a thick layer. This is a relatively quick procdure - few minutes or so should do it unless the stone is really dished badly.
When you're done with that, rinse off really well in warm water, pat dry with a towel, and let it dry out naturally.
I would not spend more than $20 on stuff to recondition it, since you can buy new ones for the same $20. Hence the sidewalk & sand recommendation.
Edit: Just noticed that Bucketstove already linked you to almost identical information. I should have read more closely. You already have the info you need.
