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:

If you are an individual adding a Personal Bank Account, select 'Individual'
If you are a business adding a Commercial Bank Account, select 'Business'
Step #6:
Follow the prompts displayed to go through the account verification process