From page 10 of the instructions

You are changing from separate to a joint return. If you and
your spouse are changing from separate returns to a joint return,
follow these steps.
1. Enter in column A the amounts from your return as
originally filed or as previously adjusted (either by you or the
IRS).
2. To determine the amounts to enter in column B, combine
the amounts from your spouse’s return as originally filed (or as
previously adjusted) with any other changes you or your spouse
are making. If your spouse didn’t file an original return, include
your spouse’s income, deductions, credits, other taxes, etc., in
the amounts you enter in column B.
3. Read the instructions for column C to figure the amounts
to enter in that column.
4. Both of you must sign and date Form 1040-X