I'm going to take Grade 6 theory quite soon. How do you all prepare for the general questions (Q4 and Q5 for 25 marks question, Q4 for 50 marks question) for Grade 6 Theory?
Is there any textbooks or guidebooks to prepare for those general questions?

Thanks.

Have you used the workbook and worked through past papers?
You will need to know the chords specified for the grade, something about all common orchestral instruments (including the german french and Italian names for them).
You need to know the transposing intervals of all instruments as you will not be given the interval when asked to transpose a passage.
Those are some of the things you need for that grade.