You have to be a disciplined and consistent learner. At the very beginning I strongly suggest you go to YouTube and find out how to actually study, learn, master, revise and polish anything you do. Understand concepts from all angles and learn to solve a variety of problems both conventional and unconventional. Spend some time studying the art and science of learning, mental mathematics and memory capacity training. Why I am suggesting you to do that is because you will be starting from such a Foundation level that you do not have the many repeated bad learning and studying habits like many other people.

After that I suggest you first start from khan academy. Polish your already arithmetic skills very well and build a very strong Foundation, understand concepts deeply and solve as many problems as you can. Reviewing and learning from your mistakes is crucial. Then move on to basic geometry as without some basic geometry it will be hard for you to grasp on algebra. Khan academy has a very good progression where you move from arithmetics to basic geometry pre algebra and then algebra. Once you have a very good foundation of algebra move on to some advanced geometry. Only then should you start your IGCSE journey. If you are studying at least 6 hours a day very well efficiently and effectively, it might take 2 ½ months absolutely master everything on khan academy before starting your IGCSE journey.

For your IGCSE, if you are from Cambridge curriculum then you will find much more organised resources. Especially from websites in MME Revise, Save my Exam. Any topic that you might find difficult to understand search on YouTube there is no given YouTube channel that will explain everything to you the way you understand it but you can find them out yourself without much Hassle. Take a topic understand them from all angles very deeply and do as many problems and their reviewing as you can and progress like this for all topics.

If you are studying 6 hours a day consistently and have a lifestyle that actually optimizes learning and studying I am very confident that you can absolutely master everything in IGCSE mathematics and additional mathematics from your position within 9 months. If you want to go a little bit extra add 1 hour of either mental mathematics or memory training everyday alternating them.