How do you tan with salt water?

If you mean laying out in the sun and using salt water to aid in your tanning process, then this is what you do.

If you are at the beach, just lay in the sun and periodically go in the ocean, don't dry off, and keep laying out.

If you aren't near a beach, get a spray bottle and fill it with water and a bunch of salt and continuously spray that on your body as you are laying out.

The salt does help you get more tan.