I did a bit of research here. As with above, you should confirm this on your own, but it looks like, to qualify for a second PPP loan, you must show a 25% reduction in revenue from 2019 to 2020, for either any quarter, or the entire year.
I was having trouble figuring out whether to count the quarterly difference based on hours worked/invoiced or check/deposit dates. It looks like it depends on the method of accounting you use. Most sole proprietors probably use the Cash method (check which box you checked on your schedule C) and that would mean you use check/deposit dates. More guidance may be released to specify which method of accounting you can use, or if you can choose whichever one you prefer.
I also read that, if you didn’t get a loan in the first round, you should be able get one now without showing the reduction in revenue.