Mark it so you can find it again. If you can't find an entry point, see Step 5. Don't remove the food until after Step 3 because ants will scatter. They are easier to kill in a line. Clean up lines of ants with a vacuum, or spray ants with soapy water and wipe up with a sponge.
Soap washes away the chemical trail ants follow.
Next, block entry point temporarily with a smear of petroleum jelly or a piece of tape. Use silicone caulk to permanently close cracks in walls, along moldings and baseboards, and in gaps around pipes and ducts. Place the bait station on the line the ants had been following. If ants are nesting in a potted plant, move it outdoors. Water it thoroughly and place it in a bucket filled with water that comes an inch below the rim of the pot.