Wiki link. But I'll give you an abridged version.
First you need to set up a
DMM account, I just used the feature that ties it into my Google account, seems to work just fine.
You then need to set up a VPN to be able to launch the game and select a server, however, server slots are restricted by what's called a lottery.
I used
softether(you need the VPN Client) to VPN into a Japan, super easy to use, just double click "VPN Gate Public VPN Relay Servers" and choose one that's in Japan. Also, set your Windows time to Japan time for the lottery.
They hold a lottery every day or two, you'll have to
stalk their twitter to find out when that is. For example, one of their recent tweets shows that it will be on the 16th(today) at 8pm JST. You must be VPN'd and registered/logged in on DMM to be able to enter the lottery. To enter, just open up the game
link here, you'll see a giant red button and that'll fire up the flash game.
Roughly 10 minutes before the lottery starts. begin joining the game. You'll get a screen of either all servers showing full, or a screen with a cat on it and some text. The latter shows you have failed to get a lottery slot(but try again!), so refresh the page after 3-5 minutes. It took me about 20-30 minutes to be shown a server that isn't full, and don't refresh faster than that or else they'll be less likely to give you a spot.
Once you've joined a server and the game is running, you don't have to worry about using a VPN any more, use the inspector in Chrome(Ctrl+Shift+i) to find your api link which you can bookmark to launch the game (it's the URL with an IP address and api_token in it).
Yep, what a pain. But all you reaaaally have to do is VPN and navigate to a page+refresh at a certain time.