Hi there

In regards to your question, the answer would be simply that you have to first make a through assessment of what all is unclear from class 11th, now there are some parts which should actually be clear before attempting to read class 12th concepts though that being said their unrelated concepts as well. Though in class 12th the content of syllabus is relatively quite less than class 11th hence it's upto you to make good use of that time and cover the respective conceptual backlog.

Hope this answer helps; best wishes