That depends on the type of income/business you client has.  See this article for more info.  Even though the layout of these screens may have changed somewhat this year, this article is still a good reference.

https://proconnect.intuit.com/community/help-articles/help/how-to-enter-and-calculate-the-qualified-...

You don't "enter the deduction" because the software calculates it. You enter the income.

Schedule C at the bottom of the w/s has info needed.

Schedule E and K-1s also need input to get QBI.