How can you separate salt from water through boiling?

At the simplest, there are two steps;

1. Boil water

2. Collect the steam

In practice, there is a little more to it. It is difficult to collect water vapor, or steam; it is much easier to collect liquid water, so you will need a way to change the water vapor back into liquid water. You do that by condensing the steam back into water.

This turns out to be pretty easy to do; water vapor will condense into liquid on the outside of a cold glass if there is ANY water vapor in the air, as everybody who has held a cold drink on a hot day already knows.

If you are out camping, it is easy to make a "solar still" and get clean, fresh water from any kind of damp muck. Dig a pit in the ground about 18 inches deep where the sun can shine on it. Put any sort of damp material - green leaves, mud, dirty water, almost anything wet - and put it in the hole. Place a clean empty container such as a metal pot or bowl in the middle of the pit. Cover the pit with a plastic sheet; a tarp, a plastic trash bag, something to cover the entire pit. It doesn't have to be airtight or watertight. Place a small stone in the plastic sheet right over the pot.

The sun will shine into the pit and heat up the wet stuff in the pit. Some of the water will evaporate. The vapor will hit the plastic sheet, and condense, and run down to the bottom point in the sheet where the stone is, and fresh water will drip into the pot.

If the goal is to collect the salt, that's even easier; put a shallow pan of salt water in a the sun. Wait until tomorrow. The salt will remain, while the water will have evaporated.