Please be aware that bookings that were in the system before the paybutton was activated have the CC info under the "Original card" field and need to be manually copy-ed over to "New card" field. 
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New bookings will follow this procedure:
When you wish to charge a booking open the "Charges & Payments" tab. On the bottom, you can see the cc info.

When you press charge, this window pops up.
Under the amount the total is displayed, if you wish to charge less than the total, input it there. 
You can choose the currency type under the currency drop-down menu. 
When you are ready to complete the transaction, press the large "Charge" button
 
 
When the transaction has been completed, a Payment ID will be generated and displayed, you can use this to find the transaction with your payment processor.
Also, a refund putton will be added, which you can use to perform refunds.
When a refund is performed, a yellow line will be created when the payment has been credited.

If an error occurs, the card owner might have to confirm his identity with 3DS authentication.
Then you need to send a Payment Link. Press HERE for those instructions

Happy charging!