If you're already registered for tax you can start sending your returns online instead of on paper by signing up for HM Revenue & Customs (HMRC) Online Services. Follow the link to 'Sign up to send returns using HMRC Online Services'.
One Activation Code per service, if applicable, will be sent to you by post. Your Activation Code can take up to a week to arrive and if you are overseas it may take a little longer. The envelope will be marked 'Government Gateway'.
You must activate your service(s) within 28 days of the date shown on the letter or the code will expire and you'll have to request a new one.