Need to make sense of large-scale and complicated client problem [on hold]

I might have an incredibly large project coming up soon which is basically impossible to approach at once. It could involve several websites on different platforms and a couple of apps that are also international.

Problems they've noted: numerous design styles and patterns used across apps/sites, numerous interruptions in the user flows

The project is quite large, and I don't know where to start. I need to break it up into a feasible project, or even better a number of feasible projects.

I would like to know if anyone has ideas or strategies for making sense and giving order to this type of complexity?