The single most critical aspect of making salt is to use the most pristine water available, avoiding areas of runoff. As the water dries out, any impurities or metals will become concentrated.
Use one-gallon, plastic water bottles to collect the most pristine water available. Plastic bottles are typically easier to manage; especially if you hike or kayak to the spot where you collect the water.
Place the collected water in a large pot and bring to a boil over high heat. Boil for at least 6 minutes to kill any bacteria. Immediately pour the boiled water 1-2 inches deep in shallow, non-reactive baking dishes (glass Pyrex dishes are preferable), cover each baking dish with cheesecloth and set outside in the sun until the liquid has evaporated. Depending on the weather, this could take a few hours or up seven days. Consolidate the salt into one baking sheet and set in an oven with just the pilot light on. Leave overnight to completely dry.
If your travels take you from your destination before your salt is dry, you can leave a little moisture in the salt before returning home, where you can transfer the salt back to baking dishes, covering with cheesecloth and let dry out completely.
Transfer the salt to a decorative bowl or canning jar with an air-tight lid.
Alternative Quick Method:
After boiling the water to remove any bacteria, reduce the heat to medium and continue boiling, stirring frequently. Larger crystals can be skimmed off the top and set aside. Continue to cook until all that remains are crystals and a small amount of moisture. (One quart of water at full boil reduces in about 45 minutes; for larger salt crystals, slow to a low boil.) Finish in the oven as directed above.