Hi

If it says that you should not consider your 6th optional subject then you need to consider your only five main subject marks and not the marks of the subject which is music in your case.

So, even if your music marks is one of the best  five subject marks, you cannot use it as it is your optional /6th subject .Thus, you need to to consider the marks of only five main subjects .

You have not mentioned the examination for which you are applying but yes in general you can follow the below tips to prepare well for your examination

* First of all go well through the syllabus of your examination

*Make routine or timetable :- you can make a routine as well to avoid procrastination  , your routine should revolve around things i.e study, revise and practice

*Make short notes for each subject and Keep revising it

* practice questions

*Solve previous  year questions of your exam  :-it will let you know the examination pattern, type and difficulty level of questions,  so you should definitely solve previous year papers

*Go through  mock test papers of your exam , it will let you know your preparation level and and weak areas so that you can work on them and perform well in your examination

Thank you