HMRC updated their guidance 4 May to outline the process for applying for SEISS.
Full details: https://www.gov.uk/guidance/claim-a-grant-through-the-coronavirus-covid-19-self-employment-income-support-scheme
The scheme will allow you to claim a taxable grant of 80% of your average monthly trading profits, paid out in a single instalment covering 3 months, and capped at £7,500 altogether.