hey! I retook my levels this year and studied on my own for them. I literally retook all three but managed, this year I got A* in bio, A* in psychology and A in chem. as long as you make a plan and stick to it, it will probably be okay, but I always worked better teaching myself so this might not work for everyone.
I resat my a levels at my old school, which is way easier since they know you and you know the building/where to go, less stress on the day. the first one or two were really embarrassing since I ran into so many people I knew, but I don't regret it now. phone up your school for deadlines to sign up to take the exams, I almost missed mine. I think they were some time before Christmas, but may differ elsewhere. PHONE UP YOUR SCHOOL ASAP and get dates/info. I ended up having to pay around £450 to resit all 9 exams/3 subjects which made me cry. however, I don't know if you would have to pay this if you actually retook the year.
I started revising around 4 months before my exams and also went part time at my job, and at 2 months out I quit my job. everyone is individual, don't use my word (or anyone else's) as gospel, but make a plan dated back form the exams, specific to you. I actually knew the suff fairly well in some areas and not at all in others, it was not linear. I aimed to finish everything a month before the exams but this ended up being closer to days, so start earlier than you think you need to. think about weekly hours/how long each topic takes you. but be honest to yourself, you don't want to waste your time or money. when I tell you this was full on, I mean it, hours daily, it was miserable.
do a lot of past paper questions. make flashcards of the harder ones, as well as topic outlines, such as the processes involved in reabsorption or mechanisms and their conditions. I used Anki which told me when to look at different flashcards depending on how well I knew them (felt organised for the first time in my life). this was especially useful for biology, I was on OCR A and familiarising myself with mark schemes helped predict what random application questions wanted me to say.
also, if you don't have one, get a job, I felt a lot less useless and earned money, which is nice. this was the first time I worked full time, and I really realised how **** school was. it is literally not real, but I did go to a stupid grammar school sixth form (ew, cliquey, superiority complex, wtf) so I asked or it. also, its nice to have coworker friends who you can do things with outside of school friends who are now at uni.
take breaks, weekends off, don't burn out. control your controllable, sleep, food, touch grass etc. sleep is especially important, sounds simple but its literally my number one piece of advice.
lastly (sorry so long), think about not retaking all 3 if possible. i focused on chem and bio initially and only started psychology properly around a month before the exams, big mistake, super stressful, basically ended up learning stuff for for specific exams (how I did well is beyond me). if I hadn't had to focus on later on psychology, I would have done better in chemistry.
if you have any subject specific questions, ask. I have useful resources/advice for my subjects if anyone wants.
**check your specification has not changed. mine did, but only a little, and the new stuff did come up in the exams.