Should you already have an account for other games on DMM and a method of accessing them, you can start playing.
New players must create an account on DMM. An English DMM account is fine ("Nutaku" is not), then you will need a VPN/Proxy in order to access DMM Japanese website. (Region Cookie method does not work for account creation). According to DMM's terms of service only those who are aged 18 or above can use their service, and those aged below 20 require parental guidance.
If you get redirected to a page describing that you should register and play at nutaku.com then you need to change the language setting on DMM website to Japanese before proceeding.
In recent attempts of acquiring an account from DMM, it says that there is a problem every time someone tries to verify their e-mail. To solve this, first click the e-mail link to verify your account, then when the page has opened saying there is an error, copy the url of the error page and paste it to a new tab and then hit enter. It should say that the verification is complete. It seems the site has a bug.