To create your own maps for "Star Wars - Requiem" you have to download and install the "Galaxy Forge" tool.
Then you should update the aforementioned software with the files found here to enable additional content like 'Oceanic' planet types of the "Star Wars - Requiem" mod.
Extract the package and see the 'Readme.txt' file for help updating your "Galaxy Forge" installation.
Additional info for experienced users:
You can replace the 'GalaxyScenarioDef.galaxyScenarioDef' file of your "Galaxy Forge" installation with the one from your "Star Wars - Requiem" mod folder if the file coming with the update-package is outdated.
I think it might be better to simply download the GalaxyForge tools as you've suggested, then copy the GalaxyScenarioDef.galaxyScenarioDef file from the Requiem mod folder to the Galaxy Forge folder.
I havent' tinkered with custom maps for a while.. but I think that should give access to all the new planets, factions and bonus types. Right?
If so, it might be worth making a nice sticky thread on this topic and providing a direct download link for the latest GalaxyScenarioDef.galaxyScenarioDef file to copy straight into the Forge folder rather than making people dig around in the mod folder.
What do you think?