The fATPL, when achieved by the modular route, includes several modules. Most of these (e.g. PPL, CPL, IR) can not be done by correspondance.
So I assume you're talking about doing the ATPL exams by correspondance, right? If that's the case, then what you need to do is look at the
CAA's list of approved schools, and find the list of schools offering ATPL Modular Theoretical Knowledge courses. Those with a "DL" after the school's name can do "distance learning", which is what you're looking for. Contact each school, pick one, and start the course.
You will find that each school splits the course into modules, usually two modules but sometimes three. For each modules, you get some books to study, and have to complete progress tests at set intervals. When you're ready, you book yourself onto a compulsary "brush-up" course - usually two weeks for each module - and after the brush-up course you go for the exams.
FFF
------------