To purchase subscription - Register an account or sign in to VPN Shield app with registered login name -> tap on 'Subscription' field:
You will get a list of subscription plans.
Select prefered subscription plan and tap on it. VPN Shield will connect to Apple Store to proceed purchase. Enter your Apple ID details and complete purchase:
When purchase is completed, your subscription data will be updated corresponding amount of days.
That's all - now you can return to VPN Shield connection screen and connect VPN.