1) If you have done your basic notes on sub topics of gs syllabus , then I would suggest you to study only international relations , internal security and newly added topics of polity and economy section ( not all topics )
Science technology and environment compilations will not add much value as number of questions are less and nature of questions is also uncertain ..I believe you can manage these questions with basic understanding of what you have studies for prelims .
2) Based on my experience
a) simplicity of answer
b) linking to the demand of the question ( by addressing all sub parts, and reiterating the keywords given in the question )
c) substantiating well with facts examples and reports
And most importantly, bringing the keywords of that respective paper which helps you to score more marks . ( Of course presentation , organising an answer and bringing multiple dimensions too plays an important role )