The best way is to have it machined and threaded.
The "not so" best way is to grind/file it round and thread it.
Both ways are effective and work fine. Both ways are really easy to screw up if you don't know what you are doing. Just remember, once the material is gone, it is really hard to put it back.
If you go to the Badger site, you can download the installation instructions. With those dimensions in hand you can decide whether or not you want to tackle it. As I've mentioned here before, I will never try to discourage someone from trying something on their own, that is how we learn. Just be willing to accept the consequences if you screw it up.