Turn off the power to the circuit you're working on at the circuit breaker box for your home.
Loosen and remove the screws at the top and bottom of the light switch cover plate using a screwdriver. Pull the plate off the switch.
Loosen the two screws on the switch, at the top and bottom. Pull the switch out of the wall when it's loose. Four wires will be attached to the switch, a black and white wire on each side. Loosen the screws holding these in place and take out the switch.
Loosen the screws on each side of the new timer switch. There will be two on each side, the same as the old switch. Slide the black and white wires under the screws and tighten, replicating the position of the original wires.
Place the switch into the opening and attach with the screws at the top and bottom. Some timers require a cover and some do not. If yours comes with a cover, place that on the switch at this time.