There’s a lot of interest in creating own worldspaces out there. Here’s a post with links on how I created the Frontier heightmap. This should get you started and it’s pretty easy when you get going. Prepare to be frustrated, because it’s a great undertaking. I made several attempts when creating my worldspace, and only after five or six attempts did I end up with something I thought was satisfying, so it can be quite frustrating, but it’s also rewarding beyond description. This is only the method I used. There are several other methods out there to try as well.
First, you need L3DT. This is software that lets you produce heightmaps. It’s fun to work with and I enjoyed it a lot. There’s a free trial of the full version, which you’ll be wanting to use, and if you like it and use it a lot, I’d suggest you buy it. It’s only when you get to importing the map you’ve made for L3DT into Skyrim where it becomes tricky. You will need the TesAnnwyn application. Maegfaer wrote a lot of posts about it on the forums, though, and his posts are what helped me get the heightmap into the game. Read and follow this one.
If you’ve gotten this far and have your heightmap in the game, congratulations. You’re one of the selected few. You are Dragonborn. But then you’ll want to add distant land to this as well, because all you see at this point is the near vicinity of your character. read the pages on CreationKit.com about how to generate distant land. The descriptions there are pretty good and to the point.
Keep your eyes on the forums and ask questions. People are very helpful over at the official Bethesda forum and there’s a lot of knowledge to be gained from there. Good luck!