To login to Next-Gen, go to my.consigncloud.com. Input your store email and password. If you are a Classic user and this is your first time logging into Next-Gen, you may have to 'reset' your password. You can set it to the same thing; it's just a quirk of how Classic and Next-Gen interface.
After you've input your store email and password, if you have multiple users, you will see a list of them pop up to choose from.
Select the proper profile and input the password if you have one. You are now logged in!
NOTE: This is not the login for vendors!