you file an amended return for 2021. it's only the unused amount that can be carried back and you need income in the same category and no excess already for 2021 to be of any benefit. the amount that may actually be used could be quite smaller than the carryback amount so you'll just have to do it and then see the result. then you'll need to adjust the carryforward when doing 2023 to eliminate the amount of the 2022 carryback that was actually used in 2021. line 23 is the limit of the allowable FTC for 2021 while line 24 is what was actually taken so the difference should be the limitation.
