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." - PRISMA protocol guidelines
A 2018 study identified an association between registration of a protocol in PROSPERO and the overall review quality of its systematic review based on the AMSTAR tool. The authors speculated that the time spent planning and documenting all of the review procedures might indirectly result in more robust methods. But regardless of where you register, completing a protocol is time well invested.
Review the checklist for the PRISMA-P protocol guidelines or the PROSPERO registration form. Can you complete all the applicable items? If so, you may be ready to register. You will be expected to stick to your protocol just as if it were a lab protocol, so make sure you have documented the process you intend to follow.
Prepare to answer the following:
Here are some of the standard protocol templates. If you are conducting a review according to a specific standard, you must use that standard's protocol guidelines. If you are not writing to a specific standard, you may still use a standard's guidelines if they are appropriate to your topic, or use PRISMA-P.
If any of the elements are confusing or you're not sure how to do them, you can schedule with a librarian to discuss it further.
Where you register your protocol depends partially on your type of review and your topic. Here are some of the options.