IIT jee preparation starting from class 12th will be challenging but achievable.since jee covers both 11th and 12th syllabus you need a strong plan to tackle class 11 syllabus quickly while keeping up with class 12th . Try to focus on understanding the concepts of physics Chemistry maths and apply them in problem solving do more and more practice. Using resources like HC Verma for physics, P. Bahadur for chemistry an RD Sharma for math can help to  build a solid foundation. For doubts solving you can take help of your coaching or online resources. Ek weekly test give mock test to know where you stand do analytics of your mock test, identify weak areas and try to improve. Time management is crucial so according to your time table you can manage both 11th and 12th topics you can start doing topics on daily basis. Success with depend on your dedication and consistency. With the disciplined approach cracking the jee starting from class 12th is possible.