Switch off the power to the area you're working at the electrical circuit breaker of your house.
Remove the screws holding the face-plate. Set aside the face-plate and screws. Remove the screws holding the switch or outlet in place -- if there's one already there -- and pull it out of the junction box so you can see the terminals attached to the wires.
Loosen the terminal screws and remove the wires. Inspect the wires for any breaks. If there are any, cut off the broken part and strip the insulation from the last inch of the remaining wire. Bend the bare metal of each wire into a hook shape.
Connect the black wire to the brass terminal of the new switch outlet combo. Connect the white wire to the silver terminal. Tighten the terminal screws. Push the switch-outlet into the junction box and secure it in place with the screws that held the old device. Replace the face-plate and secure it with the screws. Restore the power to the circuit. The switch will now control the power to that outlet.