Are you a member yet? Membership is open to all and is the first step towards accreditation.
To learn any skill, you have to practice it. As a trainee CBT therapist, ideally you will be on placement during your training. For accreditation, you will need to have completed 200 clinical practice hours that are supervised by a BABCP Accredited Therapist or equivalent.
Some courses provide the placements (e.g. IAPT courses), others you will have to find your own placement.