Turn both the gas and electric supply off to your oven. Remove the "floor" of your oven by lifting it up and out of position. The ignitor is usually at the back, and in the center. Two electrical wires come out of the back of the ignitor and are connected to two wires from the oven with ceramic nuts. Remove both nuts by hand.
Unscrew the ignitor's metal cover with a screwdriver, and pull out the ignitor. Screw into position the new ignitor and connect its two wires to the two oven electric wires (use the ceramic wire nuts). Either of the ignitor wires can be connected to either of the oven wires.
Reconnect both gas and electricity to the oven. Turn on the oven. If the ignitor starts to glow red after a few seconds, turn it off and replace the "floor" of the oven. However, if the ignitor takes more than two minutes to glow, it is faulty and will have to be replaced, following steps 1 and 2.