oh k nothing much
It depends on your location. If you can, get in touch with your school/centre and ask them for the procedure. If that's not possible or you can't get any kind of help from there, go to britishcouncil.org.uk and find your country website. There should be webpage devoted to signing up for the next examination session for private candidates (do check beforehand since the deadline is usually around the results for the previous session).
Also, make a decision about whether you want to sit in the exam separately from your A2 exams or you want to give a composite Mathematics exam. Here, the regular exam session is May/June. One of my friends repeated her AS components in the subsequent October/November session and her A2 components in May/June next year, though I would recommend you would give a composite Math exam in your regular exam session. Usually in such cases your center would take responsibility of getting you signed up and you can concentrate on your A2 exams.