When you subscribe, you'll get access for one month from the date you enter, and then you will be charged monthly for this amount. For example, if you enter on the 14th of a given month, you'll be charged every 14 of each month.
If you're using Paypal, you'll receive a notification from Paypal with your recurring payment settings, and you will be charged monthly.
If you're using your credit card, you'll be charged monthly, I'm using Stripe for this.
You don't need to do anything specific each month, it will be handled automatically.