A subscription, although an item in the catalog, is considered a service that generates recurring revenue. So, the process of invoicing and collecting payments for subscriptions is identical to that of service contracts.
To collect payment on a subscription, go to the "Payments" tab in the Subscription details view and do the following:
Generate the payment schedule for the subscription.
Use the Invoice Creation automation to automate the process of sending invoices on the billing dates or send invoices manually.
Use D-Tools Payments or supported third-party payment solutions if you want to collect payments from within Cloud.
Use the Auto Pay feature available with D-Tools Payments to automate the collection of payments on due dates. D-Tools Payments also has other automation features to further ease the accounting processes.
Check out this collection of articles about collecting payments for service contracts for all the details.