A guitar’s action is the distance between the strings and the fretboard. Action setups make sure that that strings produce a good sound, don’t buzz, and are comfortable for playing. An action too high makes playing the guitar difficult and might need lowering.
To lower the action, first check the state of the truss rod. The truss rod is an inside element going down the length of a guitar’s neck to make it slightly bent upwards. To see whether the problem originates in the trust rod, check the neck’s straightness.
There are two ways to do this: you can either take a look at the neck from an eye level by holding the guitar or placing it on a flat surface, or press a string on the first and last frets on the neck and measure the distance between the string and the sixth fret with a ruler (the distance should be about 0.25 mm). If your truss rod is accessible through the sound hole you will have to loosen the strings and adjust its nut or hex key slot with a tool by turning it 1/8 at a turn. If the truss rod has a nut at the headstock, adjust the nut with the strings tuned. Retune your guitar after each turning of the nut to check its action and repeat if necessary. Don’t turn the screw all the way round to not damage the guitar.
The next step is to fix the action by filing the guitar’s nut. For this, you will have to first tune your guitar and measure the action for each string at the first fret with a feeler gauge. At the first fret, strings are supposed to be around 7.5 mm away from the fretboard. The feeler gauge that fits the space between the string and the fret without moving the string is the action. To fix the action at the nut start by loosening the sixth string and using the proper file to file the nut. When you’re done tighten that string and check if you’ve solved the problem. If you have, repeat this on all strings.
To finish the process of lowering the action you’ll have to sand down your guitar’s saddle. You will first need to check the first and sixth string action at the bridge with a ruler. The distances between the strings and the twelfth fret should be 1.5 and 2.3 mm respectively. If this is not your case, you can lower the action by loosening the strings at the bridge and removing the first three strings to take out the saddle and sand it down.
Make sure to sand it down evenly by marking the sanding level with a pencil. Put the saddle and bridge back to their positions once you’re done with this. Then, check your action to see if the problem is fixed.
Depending on the bridge type there might be some differences in the adjusting procedure. However, the overall steps remain.