Step #1:
Log in to your Checkbook.io account.
Step #2:
Go to "Settings" on the bottom left-hand side of the page.

Step #3:
When in Settings, click on the "Accounts" tab.

Step #4:
When in Accounts, click "Add Account".

Step #5:

Follow the prompts displayed to go through the account verification process
Step-by-step guide on verification: