you will file your 2020 tax return and as part of that return there is a 'stimulus settle up' section. If you are due additional money it will appear on Line 30 of Form 1040. Everything is now based on your 2020 income and filing status; 2018 and 2019 do not matter.
if you are doing it yourself, then look to line 30 and the worksheet that will be provided to calculate it,
Why are you doing it yourself? it can be complex and an error can cost more than paying someone to do it!
Depending on your income level, you can use a number of the websites that offer to do tax returns for free. Also, there are local VITA and AARP sites in most communities that will do it for free.
ps -when you stated you are 'doing it yourself', I took that to mean (maybe incorrectly, you are doing it with pencil and paper!)
The stimulus checks are an advance on a credit you can receive on your 2020 tax return. If something went wrong or you did not get the correct amount of both stimulus payments by 1/27/2021 then you can get it when you file your 2020 return in early 2021—if you are eligible. It will end up on line 30 of your 2020 Form 1040.
At this point, we do not know yet how long the new stimulus bill will delay the filing season due to changes to the 2020 Form 1040, software programming, software updates, etc., etc. Do NOT be in a hurry to file your 2020 tax return until more is known. Give some extra time to the IRS and the software programmers so that the software can handle the new 2nd stimulus, so that you can get it all by filing ONE time.