I recently done A Level maths with AQA as a private candidate and got an A from it whilst doing full time work (30-50 hours a week), so I think I am qualified to give an opinion or 2.
You will need to select an exam centre from the AQA website, and contact the exam officier at your chosen exam centre. I had a bit of issue when I was looking for my exam centre because a number of them chose not to respond. If they do, they tend to be really slow. So you might need to persist with some and be really patient.
When paying for your fees, you will need to pay the exam centre, not AQA. If you pay early, you should be able to get your exams booked for roughly £120 (both admin and exam fees). If you pay later, this can mount up to as high as £300-400, including late fees.
In terms of study, you can choose however you want to study. You can buy the textbooks and study yourself, or go through a course with an online college like I did. The ultimate aim is to get through the exam with flying colours, and that is independent on your method of study.
I can't opine whether you should go through an online college or not, because you could easily go through a textbook and get high grades instead of going through an online college. I don't know your methods well enough. If you need me to, I can describe my experience with my online college.
Having said that, the material that they present to you via online college can significantly reduce the amount of reading and note taking that you would need to do if you do have a textbook; organise the material in a digestable way; offer tutor support where needed; and they do offer assignment questions. There is value there in studying through an online college.
With the hours and variable timeframe of my work schedule, I aimed to do 30 hours a week on top of the 30 hours of work. I maintained that I would not exceed more than 60 hours of week of combined study and work. I don't know your personal circumstances, so I cannot say whether this would work for you (no pun intended). If you have your weekends spare, it might help to serve as an opportunity for more intensive study.
The standard A Level is to be said to require roughly 300 hours of study, at least on paper. In practice, I tend to go for 600 hours because I want to secure certain target grades and become more proficient in the subject that I am studying. If I needed to, I would have done the A Level within 6 months (and I almost did during the pandemic, but I was late in applying for exams). At least 100 hours of it was spent on revision and exam practice.
The grade boundaries for B, A, and A* were 138, 171, and 220 out of 300 (or 46%, 57%, and 73%) in Summer 2022 (notice the large jump in grade boundary between A and A*). I don't know what the boundaries will be in later exams, but if they stay relatively consistent with the prepandemic levels, these would serve as good benchmarks for any practice exams you do. If you feel confident that you could score at least 60% in the exam papers on average doing 2-3 hours a day for a few months, then sure it's doable. If you're not that confident, then I would probably be more careful about how you schedule your study.
Should you want information on picking online colleges to study A Levels from, let me know. I have a spreadsheet saved when I was compaing and searching for these colleges.