The Law Society has issued guidance with a Q&A of 20 questions.
Here’s the link: https://www.lawsociety.org.uk/topics/property/q-and-a/hmrc-mandatory-tax-adviser-registration
Create with them an agent services account by 18th May, or if you have one, on 18th May make sure that your account meets the new registration conditions. Firms will have 3 months from 18th May to complete their registrations. Registration is free. HMRC are creating a video. If you don’t register, you may no longer be able to communicate with HMRC.