Install a 1/2 inch diameter drill bit into a drill. The bit needs to be at least 3 inches long.
Bore holes around the trunk of the tree you want to kill. Hold the drill at a diagonal angle (45 degrees) against the tree trunk. Push in and downward toward the ground. Drill the holes three inches deep into the trunk, and space the holes every two or three inches. Make sure you drill the holes around the entire circumference of the tree.
Mix a solution that is one part water, and two parts salt. The solution will be very thick, so stir it well. Place the solution in a watering can or another container that has a spout.
Pour the salt solution into the holes you drilled into the tree. Fill each hole completely and wait for the levels to lower, then re-fill all of the holes. Repeat this process until the holes stay full. Check the holes the next day and fill them as needed.
Mix another batch of the salt and water solution. Pour the entire amount around the base of the tree. The solution will seep into the soil and will be absorbed by the tree's roots. The solution will eventually kill the tree.