You're absolutely right about the plate movement. That will come in the next step. As I have drawn the landmasses first and then the plates the movement is already predefined in some places (the green plate for example has to move away from the violet plate above) so that the coastlines actually make sense... but I still have to figure that out for all the plates. And yes ... after I've done that I can start to figure out where mountain ranges are to be placed and where for example underwater ridges might form Islands etc. In some cases I will also have to make some changes to the coastlines.