The only install instructions I've found are for installing the game directly from the main page, but they don't exactly talk about how to install if you already have the game installed for retail. I've seen a few other people say they have both the Nasomi server and Retail installed, but I'm not sure what steps to follow on the install instructions and which to skip. Any way to get proper instructions? Or is it possible I'm stupid and missed it?

Also, as a side note, does this also work with Windower? And what point does the content end? CoP? ToAU? (Sorry for my complete ignorance, I just found out about this server)

If you have the game installed for retail and its up to date, you can just start on step 11. Ashita is basically a windower that allows you to connect to private servers, so theres no need for the actual windower program. For content its basiocally up through CoP, aht urghan exsists but isnt really coded/balanced yet so its only really there for unlocking jobs and ZNMs/HNMs.