Write a protocol for your review which sets out in advance your review question and methods you intend to use. The protocol will also state where you intend to search and how you will appraise and synthesise the retrieved studies. Doing this will help you minimise bias and avoid getting sidetracked and including studies just because they look interesting.
Background to the protocol
Describe the setting and context of your research - why have you chosen and why is it important. If there are other reviews, discuss them and identify any weaknesses and how your review plugs the gap. Use data, other research to back up your rationale. Remember you are trying to show where there are gaps in systematic reviews, not the literature.
Before starting on your protocol consider the following
Background to the review
The purpose of the background section is to provide an overview of the specific area of the review, highlight the clinical problems associated with the area or question, discuss the relevant reviews within the specific clinical area and clarify the gap in systematic reviews in this area
Purpose of the Review
Why are you doing it? Have you identified a gap in the knowledge/literature?
Specify inclusion/Exclusion Criteria
These will include specific criteria on the types of subjects, interventions (or exposure), comparative group and outcomes, and the types of studies you plan to include. It will also specify explicitly what is going to be excluded. Stating this will keep you on track.
Where you intend to search - include details of your proposed search strategy
Selecting/Appraising Extracting Data
How are you going to go through the papers? Evaluate them, extract the data from them.