Product review (5 stars): Fixed an oven that took too long to heat
Our oven was taking longer and longer to reach the desired temperature, which is the classic sign of an oven igniter that needs to be replaced.  Basically, there are two types of igniters that will fit the majority of ovens.  They're either rectangular, like this one, called "flat," or they're cylindrical, called "round."  All I had to do was take a peek under the oven, where the flame comes out of the gas pipe, and there was the very obvious flat (rectangular) igniter.  This igniter replaces all flat igniters, so there's no need to worry about what brand the oven is.

First step: turn off the power to the oven at the circuit breaker box.

Important note: the actual igniter element is mounted inside a protective metal cage for a reason.  Don't touch it with anything, especially with your bare hands.  It's very fragile.

To access the old igniter, I removed the bottom of the inside of the oven with the two thumb screws inside in the back of the oven (I was, at first, tempted to try to reach it in the way that I first was able to see it, through the bottom, broiler, compartment.  This proved to be a mistake and a waste of time).  Then, I removed the spreader plate that hovers directly over the flame.  At that point, the igniter was easy to reach and in plain sight.  I snipped the wires as close to the igniter as possible, to give myself plenty of slack to work with (you can cut a wire shorter, but you can't cut it longer).  I stripped a little insulation from the ends of each wire and used the ceramic wire caps that came with the new one to attach it in place of the old one.  Polarity doesn't matter; no need to worry about which wire to connect to which.  Never use plastic wire caps in an oven.  I screwed the new one in place and tucked the excess wire out of the way, put the oven back together, and turned the power back on.

My first test run took ten minutes to reach 400 degrees, which is a great improvement.  It used to take over an hour.  I've had it in place for a month and a half with no problems.  I'm not especially skilled at repairs, but I think this repair can be done by anyone who owns a pair of pliers and has a very basic understanding of how to splice two wires.  The fact that it was a gas oven sounds scary, but the repair seemed simple, safe, and relatively quick (about fifteen minutes of actual work, plus ten minutes of deciding how I was going to do it).  Leaning over the oven door, into the chamber, required significant lower back strength, so anyone who might not be able to hold that awkward position while working should consider finding someone who can, or finding a way to remove the door from its hinges and still be able to get it back on afterward.