A protocol is a document that serves as a work plan for your knowledge synthesis. It is important to write one to help guide you through the process, even if things change along the way.
The PRISMA guidelines for the transparent reporting of systematic reviews and meta-analyses defines a protocol as a document that "describes the rationale, hypothesis, and planned methods of the review. It should be prepared before a review is started and used as a guide to carry out the review."
For systematic reviews, the PRISMA website provides several sources of guidance on writing a protocol
In general, your protocol should have the following elements:
(Adapted from: Booth, A., Sutton, A. and Papaioannou, D. (2016). Defining the scope. Systematic Approaches to a Successful Literature Review, 2nd edition.)
Once you have written your protocol, consider registering it. Registration improves transparency of research, and prevents unnecessary duplication of research.
More information and guidance on registering in PROSPERO can be found on their website.