For each quarter you hope to qualify for, you will need to calculate the gross receipts for your business in the corresponding 2019 quarter. Once you know that amount, you should then calculate your gross receipts for the quarter(s) you wish to qualify for.
After you have those two numbers, you'll subtract the qualifying quarter receipts from the same 2019 quarter receipts, then divide the result by the 2019 quarter receipts. Multiply whatever decimal you get by 100, and you will have the percent decline for that quarter.