If you have already created an account, you can follow the steps below to start a subscription, if you don't have an account yet, please follow this guide first.
Log into your account.
Select the subscription that suits your needs by clicking the “buy” button
Click, “more channels” and select, “add your Twitch channel”.
Add any other channels you wish to connect to your subscription.
Fill in your payment details and click, “confirm & pay” to checkout and start your subscription.