The easiest way to confirm a booking is to click open the booking and then confirm it by clicking the switch in the bottom left corner of the booking window. When a booking is confirmed its information can no longer be changed and its color changes into a darker color.
Besides this you can also add payments to a booking such as credicard and cash payments or payments by prepaid cards. In order to enable these you need to enable them in the App Store (Administration > App Store). Enable Payments and then define your payment methods from Administration > Payments. First define the most typical payment methods you accept from you customers, such as cash, credicard, vouchers and then define the less frequent payment methods such as giftcards.
In order to support Prepaid cards install it from Administration > App Store and then define your Prepaid cards under Administration > Prepaid by defining e.g. 10x cards or 3 month cards or whatever you want to support. When you have defined your prepaid cards you can sell them to your customers either directly from the Administration view, or through the Customer register or directly from your calendar's bookings when you are marking their payments down.
When you have defined your payments (and prepaid cards) you will see a new Pay view in your booking information popup. You can then for instance mark a booking as payed with USD 5 cash + USD 10 voucher + USD 5 creditcard.
After you've used payments and prepaid cards for a few days you will see how easy it is to define and mark as paid and how much better you can see statistics on who has paid what and how.