Summarize:

You need enough water to fill your jar. How much water you use will depend on how large your jar is. Keep the water boiling at this point. You can use any type of salt, such as: iodized salt, uniodized salt, sea salt or Epsom salt. If you plan on eating the salt crystals, do not use Epsom salt. Uniodized salt will give you the best-shaped crystals. If you want to make colored rock salt, add a few drops of food coloring. The more food coloring you add, the darker your crystals will become. Be careful not to get any of those extra salt grains into your jar. Some people find that placing a spoon into the jar first helps keep the jar from cracking. The string should be long enough to fit inside your jar, but not so long that it touches the bottom. About 1/2 to 1 inch (1.27 to 2.54 centimeters) from the bottom of the jar will be plenty. The string should be suspended in the water. If string won't stay down, tie a bead or paper clip to it. Again, the bottom of the string should not be touching the bottom of jar. If it is, cut the string shorter. If you can, try to keep the jar covered, such as with a box. You want the water to be able to evaporate, but you don't want any dust to get inside the jar, which can prevent the crystals from forming. The room temperature should also remain consistent. As the water evaporates, it will leave the salt behind. During this time, the salt will begin to come together and form crystals. Most of the crystals will grow on the strong, but you may also get some growing on the walls of the jar. It will take a few days for the crystals to grow. The longer you wait, the larger they will become. If you want really large crystals, you will need to use a crystal seed. Click here to learn how to grow and use a crystal seed. Once the crystals get to the size you want, carefully pull the string out and pour the water out of the jar. If you want to use these crystals to eat, carefully break them off the string. You can also use a butter knife to pry them away from the walls of the jar.
Boil some water in a saucepan. Stir some salt into the water and let it dissolve. Keep adding salt until no more will dissolve, and a few grains remain at the bottom. Take the saucepan off heat, and pour the salt water into your jar. Tie a string to a pencil. Lay the pencil across top of jar and dip the string into the water. Place the jar someplace where it won't be disturbed. Wait for the crystals to grow. Use the crystals.