How do I report January 2021 wash sales in the 2020 tax return?

If you realized losses in December of 2020 and your transactions in January of 2021 triggered a wash sale then some/all of your 2020 loss will have to be carried forward (the trigger is replacement shares < 31 days after the 2020 realized loss).

If you were out of a 2020 position for 30 days then there is no 2020 wash sale problem and your 2021 wash sales belong to 2021.

Sales in a given year are what your capital gains taxes care about. For whichever year the actual sale occurred, the reduction or elimination of the loss is reflected on your Schedule D. The corresponding increase in basis (of the replacement shares you bought) is not noted in your taxes until you sell some or all of the replacement shares, when it will lead to a reduced gain or increased loss on that year's Schedule D.

Note that a wash sale can involve replacement shares purchased either before or after the sale, so there are various permutations for timing of the wash sale and the sale of replacement shares.