To clean a guitar fretboard, you can follow these steps:

1. Remove the strings from the guitar.
2. Use a rag, cotton swab, microfiber cloth or toothbrush to wipe the dirt off the fretboard.
3. Dampen the tool with warm water, mineral oil or lemon oil depending on the type of wood your fretboard is made of.
4. Avoid using chemicals, scraping or steel wool on a finished or painted fretboard.
5. Dry the fretboard with another clean cloth after cleaning.

For heavy buildup, you can use an old toothbrush, steel wool or even a razor blade . Depending on what type of fretboard you have, choose the appropriate cleaner. For unsealed/unfinished necks (usually rosewood and ebony fretboards), use an oil-based cleaner such as lemon oil or another mineral oil. For finished/sealed necks (such as maple necks), use the same guitar cleaner that you would use for the guitar body.