To get an initial Skylands map, go to more world options, and set world type to "fun world gen". Then click on customize and select Sky Dimension and make sure it has biomes: vanilla + mods selected. This will give you an Overworld that is Skylands type terrain with biomes o plenty biomes in it. For a normal overworld, set world type to biomes o plenty or default.
To get to the cave world you simply build a mossy stone portal shaped like a nether portal. Then place an ender chest inside the portal to activate it.
To get to the cave world you simply build a mossy stone portal shaped like a nether portal. Then place an ender chest inside the portal to activate it.