In most cases, the easiest way to gain authorisation to act as an agent is via the HMRC website.
Using HMRC Online Services you can set up authorisations for most common business areas, including self assessment for individuals, businesses, trusts and partnerships, NI, Corporation Tax, PAYE and VAT. Before you start, you need to set up an online account, and you may also need to ask for an agent reference number from HMRC.
Alternatively, you can use paper form 64-8 and return it to your tax office. Whichever route you choose, remember that HMRC will refuse to discuss your client’s personal or financial information with you until they have received formal authorisation.