Put the bike up on a stand. Measure from one fixed point to another, such as from the rear axle nut to a point on the fender. Mark the spot on the fender with a piece of tape so you measure to the same spot. Take the bike off of the stand and sit on it with all of your riding gear on. Bounce up and down on the seat a few times to settle the suspension. Have someone else measure from the same fixed points. The differance between the two measurements is your sag. Most people run about 4 inches of sag. I set mine to 3and3/4 inches w/o my gear on.
As far as special tools go, you only need a hammer a long flat blade screw driver and a tape measure. Be carefull not to slip and damage the threads on the shock body. You tap the lock ring loose on the shock, then you loosen or tighten the adjuster ring according to what you got from your measurement. For example, if you had 5 inches of sag, you will want to tighten up the ring. If you have 3, loosen it. Its easier to adjust the rings with the bike back on the stand. Sometimes you could grab the spring and turn it in the direction that you need.
