An acoustic guitar for classical music has nylon strings stretched across the instrument's neck, over the sound hole and across a wooden bridge with a "saddle" that holds the ends of the strings. The height of the strings over the bridge is known as the action. Lowering the action can make the guitar easier to play because there is less tension on the strings, but if it's too low, the sound becomes buzzy. Raising the action can make it harder to play but it provides a more pleasing sound quality. Guitar makers try to find a happy medium in setting up a guitar, but individual players may want to make a small adjustment to their own taste.
Measure the height of the strings over the frets on the guitar neck at the 12th fret. The number should be somewhere between 3.0 and 4.0 millimetres. Note that all six strings aren't the same height off the neck. For the sixth string, a standard height is 3.2mm, while on the first string it is 2.7mm. Decide how far you would like to lower the action. The saddle is typically lowered twice as much as the amount you want to lower the strings at the 12th fret.
Loosen the strings and remove the saddle, which protrudes over the bridge and has the strings attached to it. It should slide out of the bridge. Mark or note which side is up so you replace it in the same direction after adjusting it. Then mark the amount you want to reduce the saddle's height, using a pencil. If the saddle is glued down, its removal is more complex and a guitar repair specialist should be consulted.
Place the saddle into a steel vice with flat jaws. Sand or file down the saddle, being careful to give it a completely straight edge so it will lay flat against the bridge when replaced. Replace the saddle into the bridge and tighten the strings.
To raise the action on your guitar, follow the same steps as above to measure and then remove the saddle from the bridge of your guitar. Place a thin strip of wood as a shim under the bridge and replace it.
Buy a second saddle from a guitar store that is exactly like the one you're cutting down. This gives you a ready replacement if you make a mistake. File or sand off a small amount from the saddle at first, then put it back on and test the action. Repeat if you still want to lower it further.