Sean, go back to the auto parts store or a K-Mart, Fred Meyer, WalMart, etc and get the cheapest timing light they sell in the auto parts section. Read the directions included, and you are ready to set the timing.
Changing a cap and rotor should be an annual occasion- sure they can go for much longer, but the car will run better, you will make more power, and you might even learn something in the process.
I STRONGLY suggest you buy the bougicord wires as mentioned above and replace the ones you have. The next time you go to replace the plugs, you will likely rip the end off of the wire, leaving you pretty well screwed. Save the old set in the (likely) event that this happens the next time you are gapping your plugs.
When you replace the wires, change one out at a time so you don't get the firing order confused. Same goes for swapping the cap and rotor- line up the way the new cap will live on the distributor (small notch and big notch) and swap the wires across one at a time. Better yet, label the distributor cap and wires with their corresponding cylinder. Remember cylinder 1 is at the firewall.
Hook up your timing light to the #1 cylinder wire and battery leads, as directed by the instructions that came with the light. Get a wrench handy to loosen the bolts at the base of the distributor. On the driver's side fender, you will find a sticker that says what to set the timing at... I presume that your '90 has a vacuum advance capsule on the distributor, but I don't know for sure. Typically you will disconnect that vacuum hose (be sure to plug the hose somehow!) and you will run the RPMs up to 2000 (again, typically). An assistant is handy to hold the rpms where you need them, but you can always do it yourself by sticking something between the throttle stop adjustment screw and the throttle cam itself. Takes some trial and error, but you can make it work. Once you get this all worked out, shine the timing light on the little inspection port at the front of the clutch cover to see the timing marks on the flywheel. Sometimes the marks aren't clear; if this is the case, shut the car off and bump it a few times to get the marks lined up a the top where you can see then. A little love with a wire brush will help you see the marks much more easily.
Back to timing: note where it is at, versus what is indicated on the sticker on the wheel well. Are you a few degrees out of adjustment? OK, loosen the bolts on the distributor just enough that you can twist the distributor a bit. Slowly move it back and forth until you get the marks on the flywheel to line up where they should, then carefully tighten the distributor bolts back down, watching the timing as you go- it is easy to upset it when you are tightening the bolts!
This is a super easy job, and you will be amazed by just how much smoother the car will run, the improvement in your gas mileage, and the extra power it will create!
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