Summarize:

Sprinkling the chemicals directly on the fire typically works best on campfires. Allow your fire to burn until there is a bed of red embers beneath it and the flames have died down a bit. For the best results, the flames should be approximately 1 foot (30 cm) high. Start with just a pinch to test the chemical and ensure that no adverse reactions occur. Be sure to stand back a bit as you add the powder to the fire to protect yourself.  Sprinkle the chemical on the edge of the fire rather than just tossing it into the middle. This will decrease the chance of a large and dangerous flare-up. Wear safety glasses and fire-resistant gloves when you add the chemicals to the fire. The smoke produced by many of these chemicals can be a serious irritant, especially for people with breathing problems. Wear a protective breathing mask while adding chemicals to the fire, and be mindful of which way the smoke is going. The first sprinkle of the chemical will likely not change the flames' color, so you should keep adding more until you notice a change. In many cases, it can take up to a minute for the color change to be visible.
Build a campfire. Sprinkle a small amount of the chemical on the embers. Continue adding the chemicals until the color changes.