Consider these important things about signal fires.
• The fire should be in a very visible place, so that both the smoke and light are visible.
• The fire should be in a place where it won't get away from you. The middle of dried grasslands on a breezy day is a very bad place to burn a big fire.
• Don't let the fire get so big that you cannot put it out with the means you have at hand.
• Think about contrast. Almost all natural fire fuel (vegetation) will produce a white smoke. If it is a cloudy day or foggy, no one will notice your white smoke. Throw a few ounces of motor oil, brake fluid, cooking oil or any other oily substance into the fire to produce black smoke, which is much more noticeable.