HMRC has added a few updates for the self employed grant scheme.
www.gov.uk/guidance/claim-a-grant-through-the-coronavirus-covid-19-self-employment-income-support-scheme
While HMRC will be contacting eligible claimants directly over the next couple of weeks with more specific details you can now see if you're eligible to claim and what date you'll be able to lodge your claim by using their on-line tool (about half way down their web page).
You'll need your UTR and NI number to do this and it will also give you the opportunity to set up your Gateway account if you don't already have one and to update your contact details. It's a good idea to set this up now as the website is likely to get very busy when it goes live on 13th May.
You can find both your UTR and NI number on your tax return. If you're setting up your Gateway at the same time you'll need to provide email/mobile details and you'll need to set up additional security measures to identify you using either security questions or drivers license details or your passport. The system can time out so it's a good idea to have all the details to hand.