To create a new Spring Global Mail integration in Shiptheory go to Carriers > Manage Carriers and then scroll down until you see Spring Global Mail listed. 

Click on Spring Global Mail. This will create a new integration record in Shiptheory and take you to the integration's settings page.

Enter your Spring Global Mail API key and also enter some default package dimensions. You can get your API key by going to your Spring Global Mail dashboard's profile section and searching for "API-key".

Click "Test Connection" to verify that your connection to Spring Global Mail is working. You'll see a green tick and a success message of Shiptheory was able to connect. Once you're done click save.