Bring a pot of water to a boil. Distilled water works best. Stir in the salt. Uniodized salt works best. Keep stirring (in the boiling water) until the salt completely dissolves. Add in more salt and stir in. Continue the process until the added salt will no longer dissolve--when grains of salt simply swirl at the bottom of the pot. At this point, the solution has become supersaturated with salt.
The easiest method to grow seed crystals involves pouring the supersaturated solution into a clean and smooth container (such as a glass jar). Dangle a string, or other rough object into the solution. This gives the salt something to grab onto. After small crystals appear on the string, remove the string. These are the seed crystals.
Growing Salt Crystals
Pour a supersaturated solution of salt-water into a clean container, like a jar. Pour carefully, straining if necessary to ensure none of the undissolved salt is poured into the new container. Wait until the solution has cooled before hanging the seed crystals into the solution. Then cover the jar with a paper towel or a coffee filter. Leave the container in a place where it can sit undisturbed, preferably in a cool (shaded) place where the container will not be disturbed by vibration. Check on the container periodically. Whenever salt crystals start forming on the jar, carefully remove the seed crystals (on the string) and pour the solution into a new, clean jar and reinsert the seed crystals.
Crystals grow because salt molecules in the solution meet up and interlock. As they do so, they fit together like puzzle pieces, arranging themselves in a lattice structure, thus creating a crystal nucleus. As more molecules find the nucleus and connect, the nucleus grows too large to remain in the solvent condition and falls out of the solution, eg. it crystallizes. Other molecules in the solution continue meeting up with the crystal and as they attach to it, the crystal grows. The salt crystal continues to grow until equilibrium between the molecules in the solution and those of the crystal is reached.