Essentially, you have two choices:
1) Carry back the entire amount of the NOL from 2020 to 5 years before the NOL year (2020); or
2) Waive the carryback period and carry the NOL forward indefinitely until it is fully absorbed.
See https://www.irs.gov/publications/p536#en_US_2020_publink1000177354
Regardless, you cannot start the NOL carry back in the 2017 tax year (if you choose to carry back the NOL).
