Hello
You can prepare for CBSE 12 board through offline or online coaching.  Even you can get ample resources for preparation free of cost on YouTube. If you are preparing for competitive examination of same stream then things become even more easy you can go for standard sample paper in last 2 to 3 months to get better knowledge of pattern of answering questions.
Most of the students follow this methodology as board questions are of NCERT level and it requires only coverage of syllabus followed by last touch.

Thanks.