Building out internal docs from scratch

Hey y'all, I recently joined a design studio where I primarily focus on backend and full-stack stuff. The studio is pretty small but has been around for over a decade. They've had a dev arm for a while but it has mostly been front-end. Just recently have they started to build out the backend/full-stack arm (me!).

I'm still very early in my career (I graduated just over a year ago) but this is my second role. I previously worked at a super technical startup and while it really wasn't a good fit for me, they had really good dev practices that I formed habits around. I've been at my new role for about a week and it's already obvious to me where some dev practices at my old job can be used to improve/serve as a solid foundation for studio's growing dev arm.

The problem is that the studio takes on projects for clients, so the dev process varies from project to project. How can I best create a solid foundation of processes and tools that can be used across all projects? I'm thinking about building out internal documentation (currently doesn't exist) that includes everything from build/deploy processes to style guides. Where should I begin? What should I prioritize? Any advice is greatly appreciated!