So tell us what it takes, I'm curious

Make abortion and contraception illegal, and strongly enforce those laws.

It worked with Romania in the 1960s and 1970s. Mind you, it caused a bunch of other problems to crop up, but Romania did solve their low birth rate problem.

https://en.wikipedia.org/wiki/Decree_770

Teenage pregnancy, limited rights for unmarried women, heavily surveilled birth-control usage, all that good stuff. So long as the culture is preserved! There are 14 words that basically sum it all up