Designing FLEX is basically the same as if you would be designing a standard PCB, just your stackup is different. However, I would recommend to google for some flex design recommendations or something similar (e.g. "
flex design guidelines") - there are some specific things what you may want to follow e.g. instead of solid planes use hashed, use rounded corners on tracks etc.
PS: I started working on a flex project, but Altium is still missing some stuff from flex to make it a nice video, so I postpone that project and waiting to see what they will come up with.