Becoming a herald is simple - sign up to the blazons mailing list and contact Crux Australis (the chief Herald for Lochac) to say you wish to be a herald and you'll be registered as a macer - a trainee herald. You'll be called an "extra-odinary" herald as you'll not be tied to any group.
Once you've registered your interest as an extra-ordinary herald, your training schedule and specialities are up to you. You might like to learn about voice heraldry, or learn to conflict check a device, or how to research a name. There are a number of guides you can read in the links section which will teach you the basics of this, and can ask a local herald or an online herald to give you some guidance as you learn. The forums are a good place to test your knowledge and recieve feedback from more experienced heralds.
If you want to be group herald, responsible for maintaining heraldry for your group, there is a formal process of application and approval that needs to happen. Whenever there is a vacant position or a group herald about to retire, the position will be advertised to the group and Crux Australis will choose between the applicants, considering the previous group herald's recomendation and the wishes of local Landed gentry. People who are already active macer heralds or acting as the group herald's deputy are more likely to be chosen as they have already aquired some of the skills for the job. When the decision is made Crux will officially roster the group herald, authorising them to hold the position and charging them to uphold the duties of the office and report regularly.