Build and learn at the same time. If the building part is pushed out to after all the learning, you run the real risk of losing interest when all you're doing is learning / studying.

Whatever it is you want to build, break it down into very small initial components that don't involve learning ALL the math stuff, just some of it. Then build and learn together.

But breaking down your idea or project like this isn't easy. It's a skill all of its own.