Go to Profile and you will see this window and information about your trial end date, but you can start your subscription while still on trial, if you want.
Click `Start subscription` and choose monthly or yearly subscription. As you can see, yearly subscription is a better deal.
Write down your card details and click `Pay`. (Payments are securely handed by STRIPE.)
Your payment will be made straight away and your subscription will start.
Subscriptions will be automatically every month/year at the same date.
You can cancel your subscription any time. Go to User guide - Cancel subscription to see the steps on how to do it.