After signing up for the free trial, you will be lead to a dashboard that will look like this:
a. Click on the Plus icon (+) link beside the WooCommerce & QuickBooks online.
b. Click on Connect.
c. Login to intuit using your username and password.
d. Choose a company (if you have only 1 company proceed to step e):
e. Click Authorize.
f. This will finally lead you to the page that will say that you have synced in your QuickBooks online and Woo successfully, and you will be given an API key which you may find in your dashboard and copy it: