To do that, you should open control panel, and open "e-Payment Gateway" menu:

If you have any unpaid documents, you'll see a "Subscribe" button on all unpaid documents.

To subscibe on PayPal recurring payments for any of your current servers, you need to press "Subscribe" button, and proceed with subscription.
After pressing this button you will see a short summary of your upcoming subscription:

If you wish to continue, press the "Proceed with subscribe" button. You will be redirected to the PayPal website.
You need to input your PayPal account credentials, and press "Log in" button to continue.

Then you will see the details of your subscription. If you wish to accept your new subscription, you need to press "Agree and Pay" button.

If your subscription was created sucessfully, you will see page like this:

It means that your subscription has been successfully created.
After same number of months, as your server contract has (1,3,6,12), PayPal will initiate new transaction for your subscription automatically.
You can cancel your subscription at any time, please read instructions here.

Important: PayPal takes additional commission fee of 3 € in total for each recurring transaction.