You'll want to start by creating a new mod from an Empty map template.
(refer to Creating New Mods for guidance)

If you are trying to migrate an map project from ASE, exporting the landscape heightmap from the ASE modkit can save you some time.

ARK: Survival Ascended uses World Partition for map development. In fact we don't support world composition anymore at all.

To get started select File -> New Level -> Open World
And then File -> Save Current Level As...

Save the level to your mod plugin directory.
IMPORTANT: Make sure not to accidentally save it somewhere else and try to move it later. wherever you save the world partition level is where it should stay. Moving World Partition levels requires the use of commandlet tools because there are many hidden files that won't be moved with it otherwise and these files NEED to be in your mod plugin.