There are many ways to create a map, and it sounds like you've found instructions which use SpringMapConvNG.
SpringMapConvNG is now part of the standard spring build and release process. You should be able to grab the archive corresponding to your operating system at eg
https://springrts.com/dl/buildbot/default/develop/104.0.1-2031-g3397b1e/ - anything that ends with portable.7z should be fine, if there are multiple versions available.
Once you've downloaded the archive, you should be able to find binaries called mapcompile and mapdecompile at the top. These are the SpringMapConvNG binaries. You can use mapdecompile to inspect existing maps for examples, and mapcompile to create maps of your own.