An account can be activated any time during or after the trial by clicking on the activate account link in the header or by navigating to Options (⚙) → Plan Info → Activate account. Enter your credit card information to begin the billing cycle for the current plan tier. The credit card will then be charged the monthly amount.
All data that has entered during the trial period is preserved when activating an account. Each month, when the anniversary date comes up for the account, the card on file will be charged and the owner of the account will receive an invoice via email showing the charge.