To utilize BYOND's subscription system all you need a publicly listed hub (fan-games cannot use this system).
Once you have one of these, in the hub entries' settings there's a place where you can set up subscriptions and their cost and length. Your hub will be assigned a subscription passport code that you'll need to use with client.CheckPassport().
After that subscriptions are automated. The player subscribes via the hub using Paypal, BYOND takes out their share, and the rest is deposited into your BYOND Wallet (from there you can extract your funds into your Paypal account). Hub subscribers are added to a list kept by the hub.