I simply make a new umap file, make it active and create a new landscape, I then unlock it and move it where i wanted it (making sure it overlaps slightly for “welding” later) then i start shaping it like i want it, making sure it kinda continues where the other landscape stops (it is alot easier to hide the “weld” in flat area’s)
There is a second way which i have not done as i discovered it way later.
You can use the add tool (under landscape) and simply keep adding to your landscape to make it bigger with this feature you can actually make your landscapes 10000x10000 instead of just 32x32 (do note max umap size for cooking is 600mb) Once you got the desired size (make sure its a perfect square) then use the move tool to divide the landscape up to smaller pieces (either 2x2 or 3x3 etc etc)