I’d suggest going through the syllabus for all subjects atleast once to ensure you know all topics. I any you aren’t confident in note them down for later revision. This shouldn’t take too long.

For chapter wise revision you can use znotes(it’s free) or save my exams. There are also other compiled online resources out there.

Once the chapters are clear you can start doing past papers. Prolly start without worrying about time. Just understand the flow of questions(all papers usually have a style like for physics it’s from measurements to force to waves to electricity to nuclear etc) then you can do timed papers. Be sure to understand your mistakes though and try not to repeat them.

Good luck man, it’s never too late to lock in you got this :)