To master the Guitar in Bitlife, players will need to start practicing it when the option unlocks at age eight.
When player's characters turn eight, they will be able to start practicing the guitar by visiting the Mind and Body tab and then clicking instruments.
Under Instruments, the option to practice guitar will be located, click practice guitar, and you should be able to start lessons if your parents allow it.
Once you start guitar lessons, keep practicing every year until you raise your skill bar with guitars to the highest value, which will be a bar that is close to being fully green or fully green.
Related: How to master the Piano in Bitlife
You can raise your skill by practicing around once or twice per year, any more, and you will not raise the level. That said, keep practicing the guitar every year until you graduate from high school.
By the time you graduate, you should master the guitar, but if not, you can always start lessons again to continue practicing or start a career as a solo artist via the special music career tab.
Private lessons or a music career will both allow you to practice your music skills beyond high school and earlier. But with enough practice and skill, you should have already mastered the guitar before you even graduated.
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