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 "Profile Info" tab.
Step #4:

When in Profile Info, look for the "Integrations" section.
Step #5:

In the Integrations section, press the "Connect to QuickBooks" under the Quickbooks logo.
Step #6:

You should then see a window that looks like the above. Press the "Connect" button in the bottom right. Proceed through the remaining process to finish connecting your checkbook account to quickbooks.