Not very hard at all - gaskets will run you roughly $25 (three to get for one side), unless you get all the little rubber ones for each bolt, then add another $25 to that total.
http://www.subaruparts.com/diag/?mod...category=020-A
(Note: the gasket's aren't the same from one side to the next, so be sure you get the ones for the correct side of your car)
The work can easily be done while the engine is in the car. By left side, you're referring to the driver's side correct? Then all you will need to do is remove the battery and its plastic case, and you'll have enough room to work with. They're ten bolts to undo which you can see in this pic of my engine pulled (you may also have to unbolt the oil filler tube - not 100% sure but very easy to do):
From there, the valve cover should come off easily and you see the beautiful camshafts inside:
Clean out the inside of the valve covers, remove the old gaskets and replace with the new. Put valve cover back on (set torque wrench to ~10-12 ft-lbs) and you're done.
As you're doing this for the first time, may be a 3 hour job, but take your time and do it right. Also, DON'T let any debris get into your engine when you have the valve cover off. If a small amount of dust or a fragment of a rock gets inbetween those cam teeth, you're screwed!
-Chike
