For the most accurate representation, you will need to obtain a DEM of your area
This is not something that can be made very quickly by a beginner, it involves various operations with several programs.
The DEM needs to be processed in a program such as Microdem, then it needs to be converted to a terragen file and loaded into Wilbur for final conversion to a Lon/Lat text file.
This can then be imported into WrpEdit.
This is only one way of doing it, there are many other methods, programs, and of course WrpEdit is not the only tool for making islands.
Planck