In the BigCommerce app marketplace locate the Shiptheory app.
Click the Shiptheory app and then hit 'install'
This will then show you a confirmation page. Once you are ready, hit the confirm button.
If you are currently not logged into Shiptheory, or don't have an account you can either sign in or create an account and sign in:
You'll then be directed to the activation settings. Here you can specify when you process orders, ie, when a status update will trigger the shipments being sent over to Shiptheory.
Once selected, click 'activate'
You'll then see a screne where you can specify further settings, including the which status you want Shiptheory to process orders at.
When done, hit save.