Capital gains stacks on top of your regular taxable income.
Suppose your taxable regular income (after all deductions) is $30,000. Then, the first $18,350 of capital gains will be taxed at zero percent, because your total taxable income is under $48,350. Every dollar of capital gains above that will be taxed at 15%, because your total income puts you in the bracket where regular income would be taxed at 22% and LTCG are taxed at 15%.
