Log into your account and navigate to "my account"
Click on edit subscription and you will see your current subscription with your other channels connected and an "add instagram account" button which you should press.
You will be directed to connect your Instagram, please ensure that you are either logged in or login to the instagram account you wish to link and press the green authorise button
You will be redirected back to the subscription page where to finalise your subscription you need to click "Save changes" as indicated in image provided.
If you accidentally add the incorrect channel, you will need to cancel, resubscribe and contact firstname.lastname@example.org to resolve any payment issues.