Turn off both the main electrical and gas supply to your oven. Remove the bottom metal plate of the oven by pulling it up and out with your hands. Remove the wing nut holding the gas burner cover in position, if applicable. Locate the ignitor at the back--it has a metal cover, and two electrical wires coming out of the back.
Remove the two ignitor wires from the oven wires by unscrewing the two ceramic wire nuts by hand. Unscrew the metal ignitor case from the oven bottom, and take it out. Screw the new ignitor into place and attach its two wires to the two oven wires, using the ceramic nuts included with the ignitor; either ignitor wire can be attached to either oven wire.
Turn your main gas and electrical supply back on. Turn on the oven. The ignitor should start glowing in a matter of seconds, but if it takes more than 2 minutes, the new ignitor is faulty. Replace the gas burner cover, as well as the bottom metal oven plate.