Locate the breaker box in the house. Find the circuit breaker that supplies electricity to the room you're working in. Flip the breaker to the "Off" position.
Use a non-contact voltage tester on the first outlet or switch to make sure the electricity has been shut off.
Use a screwdriver to remove the screws in the outlet cover or switchplate; set them aside. Remove the cover plate.
Put on gloves to protect your hands. Fill the gaps all the way around the electrical box with acrylic latex caulk if they are smaller than 1/4 inch wide. Fill larger spaces with foam sealant that's designed to be used around doors and windows because it won't drip. Trim off any foam sealant that sticks out past the wall after it dries.
Place a pre-cut foam gasket on the electrical outlet or switch to block the draft. You can easily look at the one-piece gasket and see how it fits. It simply slips over the outlet or switch.
Replace the cover plate and screw it back on. Repeat the process with the remaining outlets and switches in the room.