A protocol is a plan or guide outlining the steps that will take you through the Systematic Review process. Protocol guidance can be found in PRISMA 2020 statement (for medicine and health-related disciplines) or from ROSES (for environmental science related disciplines). This includes checklists, flow diagrams etc. Ramses is an alternative to PRISMA for realist reviews.
This BMC Medicine article "RAMESES publication standards: realist syntheses" discusses realist synthesis as an alternative systematic review method.
This article "ROSES RepOrting standards for Systematic Evidence Syntheses: pro forma, flow-diagram and descriptive summary of the plan and conduct of environmental systematic reviews and systematic maps" discusses the applicability of PRISMA and ROSES models for use in different disciplines.
Your protocol needs to include the following and its aim is to keep your SR on track and focused:
Check the following resources to see what best fits your needs:
Article: Best Practices in Systematic Reviews: The Importance of Protocols and Registration
Further reading: Planning a Systematic Review: think protocols a blog post by BMC on the benefits?
Covidence/A practical guide to Protocol Development for Systematic Reviews - this downloadable guide is a comprehensive resource for crafting systematic review protocols.