How do you become asexual?

Typically, you can't "become" a sexual orientation. You have to be born that way. What a person can choose is abstinence or celibacy. Celibate persons are typically fighting through their desires, while asexual persons lack such desires.

Some hypothesize that brain injury and personality disorders can cause the asexual orientation in some individuals. But such thinking should not be applied to asexual persons as a group.