Start by going to an existing Event Page or create a new one. Move on to your Event Settings step and scroll down to the bottom where you'll see Advanced Settings.  Under this category, you'll find a Subscription/Flex Pass Event toggle. Click this to make sure the toggle says YES (see image below.) 

 Next, move on to the Event Setup step. This is where you can add a ticket level to represent your subscription. To do this, click on the Edit button in the corner. 

After you click Edit, a screen will pop up with your ticket level options (see image below.) This is where you will give your subscription a name, price, etc. 

At the bottom, you will notice you can make the subscription available for an unlimited number of shows by moving the Unlimited Tickets Allowed? toggle to YES or you can set a limit for how many shows the subscription can be used for by entering a quantity in # of Ticket Purchases Allowed in Subscription. For example, in the description, you will see the 12 tickets included in the subscription which will cover a family of four for three shows. 

Lastly, once you move onto the Customize Ticket step, this is where you will design your subscription ticket. The customer's subscription level, number and number of shows will be included by default.