As far as the checking of back/carry over papers is concerned, there is no major difference between that of regular papers. It is just a myth that lesser marks are given in back papers than regular ones. In fact, there are many cases where the students scored higher marks on back papers in comparison to their regular ones.
There are some tips you can make use of to get the best out of back papers:
For theoretical answers always begin with a heading and definition. Proceed with answering the question in points (especially 10 marks questions)
Don't forget to include neat diagrams wherever necessary
Don't write extraordinarily long answers and refrain from including irrelevant information.
Done leave any questions unanswered. Write whatever little knowledge you have related to the question. You might get 2 or 3 marks for this.
If you are unable to go through the entire syllabus, make sure you study the previous 5 years' papers deeply. Almost 50% of the paper is from repeated questions.
The key is to input maximum effort. Start studying at least 10 days prior to your exams to get good results.