A protocol documents the key points of your systematic review. A protocol should include a conceptual discussion of the problem and include the following:
It's also worth looking at example protocols in the PROSPERO database to see how they were written.
If you are undertaking a lengthy systematic review i.e. 1-2 years it would be a good idea to register the protocol in the PROSPERO database so it can then be peer reviewed and to reduce unnecessary duplication of effort among researchers. If your review protocol has been registered you must indicate where it can be accessed, i.e. give the registration number & web address, or describe if you have had the protocol looked over by others e.g. AHRQ or people in your institution.