To claim your NOL carryback, you'll need to file an amended 2016 and/or 2017 tax return using Schedule X, Explanation of Amended Return Changes. This will allow you to claim an additional refund amount up to your tax liability of the year you're carrying the NOL back to.
You must first complete and file your 2018 tax return and attach Form 3805V, which shows the allowable carryback amounts. After your 2018 return is filed, you may file the amended returns for the previous two years.
Enter the carryback amount being used for each year on Schedule CA, Line 21d or Schedule CA-NR, Line 21d. You should provide the following explanation of changes on your amended returns: "2018 NOL carryback deduction". Do NOT attach your 2018 Form 3805V to your amended returns.