Took my 2001 Tacoma 4X4 TRD with 130K to the dealer to have the shocks replaced (didn't want to hassle with the front ones) and have them check the oil leak. They said the valve cover gaskets were leaking and wanted $540 to replace them (4.5 hours). Far too much in my opinion, so decided to do it myself. I finished installing the new ones last night and learned some things along the way. Probably took a total of 5-6 hours (not counting the parts runs). It's not that hard, but takes some time.
First of all, if your valve covers are leaking the first thing I'd try is tightening the bolts you can reach, but don't crank them as tight as you can. When I took them off I was surprised at how loose some of them were. If it still leaks oil, you need to replace them.
Tip #1 - Buy all of your replacement parts at the dealer. I went to Autozone for the gaskets and the spark plug seals and the seals in the kit weren't like the originals. The originals were heavy and the replacements were flimsy.
Tip #2 - Replace the intake plenum gaskets. You'll read online that they're reusable, but at $12 each it's cheap insurance to replace them. The ones I originally ordered through O'Riely's were a different material (not metal), so I had to get them from the dealer and take the O'Riley ones back.
Tip #3 - Replace the PCV valve, valve cover grommets (didn't come with the kit), cam seals, spark plugs and ignition wires (you'll need the boots too for the passenger's side). You're already in there, so you might as well knock out any future problems before they happen. Replace the PCV valve and grommet when the cover is off. I did it when it was on and most everything was put back together, and the old grommet cracked and fell inside. I was able to pull it out, but a few colorful words were said thinking I'd have to take the cover off again.
Tip #4 - The valve cover bolts are torqued at 52 inch pounds. That's only 4.3 foot pounds and there wasn't an auto parts store in town that had one in inch pounds (NAPA wanted over $200 to order one). I wound up using the old style needle torque wrench Ace Hardware had, but it was my only option and if I were to do it again I'd order one online. If you get one from 20-200 inch pounds you can use it for the intake plenums too (144 inch pounds). You can find these pretty cheap on eBay.
Truck doesn't leak now and it's tuned up for winter. If I do this again it would take half as long, but a complete novice may destroy the engine if something gets dropped inside when the plenums are taken off. Best to block the holes with rags as soon as you take the plenum off and take pictures as you go. Also, the Toyota repair manual was next to worthless. I'm sure the information is in there in various places, but they don't even list "valve cover" in the index. The web was most useful and a search for your engine will probably tell you what you need to know.