Finishing my A-Levels

Hi all im 20 years old i went to my local sixth form until i was 18 years old. I completed my AS levels but i decided at the time to drop out and join the army which i did. Now ive been in the army just over a year and i want to finish my A-Levels so i have some qualifications for when i eventually leave.

I am interested in completing my A-levels in history(AS grade B) and politics (AS grade C) and perhaps maths aswell

I have been studying these subjects in my spare time so i am not to far behind in the required subject knowledge.

I want to know how i would go about completing these A levels sitting as a private candidate. What would the process be? How would i go about any necessary coursework? Could coursework be done in my spare time or would i need to go an examination center to do it? How much would i be looking at spending?

I would be interested in hearing anyones opinion on this aswell as any answers to my questions by anyone.
Thanks