You can find the Citi Double Cash Card login page on Citi's website. Everyone who has a Citi account can log in to manage their account there. There is no separate page for a Citi Double Cash Card login.
You can also access the Citi Double Cash Card login page from the Citi mobile app, which you can get for free for both Apple and Android. This is probably the most convenient way for you to manage your account on the go. You'll just need to log in on the app with the same information you'd normally put in to the website login field.
How to create an online account:
If you don't have an online account, it's pretty easy to create one. Just go to the login page  and click below the username and password fields where it says, “Register for Online Access.” You'll then need to enter your card number, along with your birthdate and the last 4 digits of your SSN. Then you can set up your account and create a username and password. After you're logged in, you'll have full access to your account, allowing you to view your balance, see your transactions and rewards, make payments and more.