How to adjust a guitar truss rod
Truss rod adjustment (or Neck relief)
When you adjust your truss rod you are creating a forward bow which creates relief in the neck. This is simply a way to have your strings pass over the upper frets without touching them.
If your neck is bowed backwards the strings will hit the upper frets too soon. If its bowed back even more the strings will hit somewhere in the middle of the neck which wouldn't work at all.
The very first thing you need to do if you suspect your truss rod is out of adjustment is to measure it to see if it really is. There is no point adjusting the rod if it is not needed. You could actually cause problems that weren't there to begin with.
I'm not trying to scare anyone out of adjusting their truss rod but if it aint broke dont fix it.
Don't adjust your rod if you are just trying to lower your action. The truss rod is not there for that purpose. Lowering action is accomplished with saddle and nut adjustments.
Checking your truss rod is a fairly easy and straight forward thing. I usually do this at the low e string only because I can see it better but you can do it at any string.
To Adjust your truss rod
When doing actual adjustments it is best to loosen the tension on the strings. This makes it less likely to strip the adjuster.
To start mount a capo on the first fret. Then fret the fret closest to where the neck meets the body. I don't own a capo so I fret the first fret with my left hand and the upper fret with my right pinkie and hold the guage between thumb and index finger.
Next take a feeler guage and measure the distance from the bottom of the string to the top of the 7th or 8th fret. Be sure you measure to the top of the fret and not the fingerboard. It should be around .010+- (ten thousands). This is just a starting point. Most guitars will play fine at this height if everything else is within tolerance.
If when you measure your relief you get .008 or .007 or even .006 and your guitar has no fret buzz you can leave it there.
Theoretically if everything was perfect you could have a guitar with a perfectly straight neck with 0 relief and it would play fine. Problem is nothing is perfect.
Sometimes with certain necks even .010 isn't enough to stop fret buzz and you have to introduce even more neck relief but in most circumstances .010 should be enough.
Be sure to use the correct tool when adjusting truss rods. Sometimes a loose fitting tool will damage the adjuster. If your not sure you have the correct tool or you feel uneasy about doing the adjustment it might be better to bring the instrument to a music store where they will have a qualified repairman.
If you strip the truss rod nut (or allen) the neck will be much more difficult to fix.
The two photos below show what happens when you adjust the truss rod. These are extreme examples but it shows what happens when the truss rod is adjusted in each direction.
The photo below shows no relief at all. The strings actually hit in the center of the neck. If you turned the adjuster and it looks like this you need to go in the opposite direction.
The picture below shows a lot of relief. This is what you want just not as severe as this picture. This picture is exaggerated so you can easily see the convex shape.