The only advice I can give is from my own experience: I've studied 3 of their MSc modules, at a rate of one per year whilst working full time. It's a shame their undergraduate courses are now so expensive as some would be ideal (re)preparation for the MSc. It's useful/important to know that the "routes" they describe are just suggestions: you can study any combination of the modules, as long as the dissertation is included. I've taken variational principles, coding theory and fractal geometry and really enjoyed them all. Will probably move to number theory next. The student reviews are interesting to read: I think I'll avoid the numerical course that uses Maple.
Most courses have a set text which you could buy in advance and try and make some progress through, for example during the summer months. The OU provide very helpful online tutorials and feedback on assignments is also generally very useful. There are also online forums for discussion/clarification of anything and everything in the course. Specimen and past papers help to prepare for the exams. Moreover each course comes with a kind of glossary which you can annotate and take into exams: develop the skill of condensed writing and then these can be a very reassuring resource in the exams.
In terms of more general study skills, you just need to plan how to fit the studying into your life: for me as a teacher, I would do big chunks at half term and in holidays etc. Others prefer to set aside a couple of hours every week. Particularly when you study two modules at the same time, there can be stressful points in the year if they each have an assignment due around the same time. The exams typically take place early June.