you have to file a 2020 tax return, which you can do for free if you are eligible at filefree.intuit.com. There is no separate form for the recovery rebate credit, it is simply part of the 2020 tax return,
but suggest waiting until the end of January - there are still a lot of payments flying around and fixes coming from the IRS.... may save you some work. The IRS won't accept a tax return until Jan 27 in any event.
