I am interested in completing a phd but was hoping people could give me some advice on how to narrow down my research interest, what research is needed, how to find a supervisor etc

Need more details about your research area. To be honest, we can't really provide much help. All you can do if you want to submit an application for your own project is to know your literature inside and out and find the gap that you want to address, think of the methods you need to use and then find a suitable lab to do it and apply.

How about looking at advertised PhD posts? That was how I got mine - someone had already defined the broad research area and then at the stage of applying I had to say how I would approach the topic (there was a lot of leeway to do my own thing with it, but I think this differs between PhDs).