Enabling the add-on will send all new invoices from ActiveCollab to your QuickBooks Online account for further processing. After sending an invoice to QuickBooks, you won't be able to edit it in ActiveCollab anymore.
Also, all invoices made before connecting to QuickBooks, as well as the entire Estimates section will become unavailable. However, they won't be removed and will reappear if you disconnect QuickBooks.
Open Add-Ons and click Configure next to QuickBooks.
Click Connect to QB.
Sign in and click Authorize.
Before you can connect, you'll have to provide the OAuth Consumer Key and OAuth Consumer Secret. To get those, open a developer account, create a new app, deploy it, and copy its keys to ActiveCollab. You'll also have to choose the company you're connecting to.
When creating a new invoice in ActiveCollab, you'll be able to choose a client from the list of companies from QuickBooks.
You can then select the desired projects, which time and expense entries to include, and how to group them.
To create the invoice, click Proceed creating invoice in QuickBooks.
The newly created invoice will be sent to QuickBooks and show up under the selected client. There you can finish creating the invoice and send it.
You'll be able to manage invoices only in QuickBooks - but the changes there will be reflected in ActiveCollab as well:
When you disconnect, all the invoices you sent to QuickBooks will become unavailable in ActiveCollab (but they'll stay in QuickBooks).
They'll reappear in ActiveCollab if you connect QuickBooks again.