So in very basic terms I THINK he will be paying a grant of up to £2,500 per month to self employed.
The grant will be calculated based on 80% of your profits declared on your last 3 years of tax returns but if you haven't got 3 years they will "consider what you have"
So for example, if your 5th April 2019 return showed profits of £30,000 your 2018 showed £20,000 and 2017 showed £25,000; you would add those three years together and divide them by three to get your average income e.g £25,000.
On a monthly basis that would be £25,000 / 12= £2083.33
80% of that would be paid as a monthly grant = £1,666.67.