Back up your world before use, just in case!
To pregenerate, use the /pregen command. It takes the following parameters:
- radius (Int) - the radius around which to generate (in chunks)
- dim (Int) - the dimension ID to generate (hint: 0 for overworld, -1 for nether)
- here/spawn - 'here' will generate around the player running the command, while 'spawn' (default) generates around dimension spawn.
Commands look like `/pregen 50 here 0` (to generate 50 chunk radius around player position in the Overworld, as an example)
To abort pregen, use '/pregen stop'
Note: the server console MUST specify the dim, and cannot use 'here'