Before you get started, identify the type of ant you're dealing with. The identification guide put out by Bayer Environmental Science is thorough and available for free online.
When dispensing ant poison, always wear rubber gloves, a protective face mask and safety glasses. A flashlight is handy for spotting nests under bushes or other undergrowth next to the lawn. Finally, you need your ant poison. Most come in the form of a spray, but fire ant poison requires the use of a broadcast spreader for effective distribution.
Before you begin spraying, mow your lawn. The shorter grass allows the poison to reach the soil more easily. It also makes spotting nests easier.
Spray your lawn with insecticide in the early morning or late afternoon. Ants are most active at these times. If the problem persists or returns after six weeks, spray again. Spraying won't kill every ant in your yard, which is a good thing, but it can keep them to a manageable level and destroy any large mounds.