1. To check your neck relief, place a capo on the first fret of any string (the lowest string is the easiest to measure with) and with your picking hand, fret that same string on the fret where the neck joins to the body. Then inspect the gap between the fretted string to the 7th fret. A setup that's suitable for the way most people play is about 0.5mm (≈ 0,02 in). You can measure this accurately by sliding a feeler gauge the appropriate thickness between the string and the fret. Too little gap or no gap whatsoever means that the truss rod is too tight. More than 0.5mm indicates that the truss rod may need to be tightened,
2. This method is also a way to tell if your neck is twisted or your frets have excessive wear on one side or the other. Measure the relief with the method listed above on the bass side and also measure the same way on the treble side. A perfectly straight neck calls for both measurements to be exactly the same. Any deviations indicate that the neck could be twisted.
Make a note to say however, that sometimes just looking down a guitar neck to check for twist can be a bit deceptive, because the nut action on most guitars does tend to be lower on the higher strings, and that can sometimes create the optical illusion of a twist. Measuring the relief of the bass string and comparing that gap to the amount on the treble string with a feeler gauge is more accurate.
3. To adjust the truss rod, ensure that whatever adjustment tool you use (be it an Allan wrench, a screwdriver or a box spanner) is fully engaged with the nut it indexes into. Avoid adjusting the truss rod without the tool fully engaged or you risk chewing the adjustment nut up.
4. Asian-made guitars generally call for metric tools to adjust them. US-made guitars (and some Mexican) generally call for Imperial. This is very important because a 3/16" Allen wrench is ever so slightly undersized for the 5mm barrel nut to adjust the truss rod found on many Asian-made guitars and you seriously risk stripping the barrel nut out if you use a 3/16" wrench on a 5mm truss rod. Make sure you use metric tools on metric parts!
5. Truss rods nearly always tighten in a clockwise direction facing down the adjustment nut. This means that a truss rod nut at the bottom of a guitar's neck will adjust in the opposite direction to one at the top of the neck.
6. Only tighten the truss rod up to a 1/4 turn at any one time. You can tighten it more than that, but allow time for the guitar' neck to stabilize from the forces you're imparting on it before you adjust it further. Measure your neck relief again with every adjustment you make.
7. Some truss rods can act on the neck in both directions. They're called 2-way truss rods and they're more common on modern guitars. If you guitar has one, you can turn the truss rod counterclockwise and it'll actually force the neck into relief rather than straighten it out. This can be very useful when you want to use really light gauge strings or if you neck naturally backbows with no string or truss rod tension on it.
8. If you have adjusted the truss rod to be very tight, and you're still not getting the correct neck relief, consult a guitar tech. Overtightening the truss rod can cause damage that is very difficult and expensive to repair. Exercise common sense; If you feel as though you might break the truss rod if you tighten it any further, STOP.
9. If you're about to adjust the truss rod and it's already tight, always start adjusting it by loosening first.
10. If your guitar's action mysteriously seems to raise or lower when you can swear on your mother's life that you never adjusted the action at the bridge, then do not adjust the action at the bridge. Check the neck relief in the method mentioned at step 1 first. Chances are, you'll find that the neck relief has changed, which has lead to the loss in playability. Adjust the truss rod accordingly. Adjusting the action at the bridge when only the truss rod needed to be adjusted will only compound the problem your originally had; because not only is the neck relief still not set correctly, but you've now set the string height at the bridge incorrectly too.