So it looks like 80% avg of your last 3 years books.
For upto 3 month (longer if needed)
You don't apply. The tax office will contact you to fill in a short form but not until june at the earliest.
If you qualify you can apply for universal credit until the payment (in june) is sent..
The payment in june will be a one off/lump sum payment and cover for the full 12 week, backdated to today