There is a good possibility you don’t need to include anything with the 2020 income tax return.
You said the T1198 QRLSP is for 2019, so you need to establish or confirm what year this relates to.
There had to be an income receipt (possibly a T4A) to report the lump sum payment as income. What year was on that slip? If it was included as 2019 income then nothing happens with the 2020 T1, you need to amend the 2019 T1 return to request the lump sum be allocated to prior years (unless the CRA already took care of that last year). You wouldl need to check the 2019 NoA.
If the T4A and the T1198 are actually for the 2020 taxation year then you prepare the tax return as you ordinarily would (declaring the income in 2020), but instead of efiling the return you would mail in the T1 return with a cover letter requesting an allocation of the lump sum payment to prior years (if beneficial). You can’t allocate the income, nor can you request prior year returns be re-assessed… that’s not how these work.
Canada Pension is the only retroactive lump sum payment where the issuer does not issue a T1198… however…the CRA is supposed to automatically assess the return based on whether it is more beneficial to allocate the payment over one or more years.
The first thing you need to establish is whether you have the T-slip to report the lump sum payment and what year is on the slip.