To start with you can take the stratis or altis mission files and rename them to open in the editor on your map.
For example you could download a3wasteland,Stratis (unpack the pbo) and change the name to a3wasteland.YourMap...add it to your miisions directory in documents>Arma 3 -OtherProfiles > "Your Profile> missions.
Open the Arma editor with your map as an active expansion then open that mission and you should be able to see the spawn points, stores, towns, territories and mission markers. From there you can begin to redesign Wasteland to suit your map.
I did just this with Esseker.