You as a member, need to just (1) add funds and (2) add the model on your Skype contacts list and then (3) video/audio call the model on Skype as usual.
When the call is accepted, you will be charged per minute. The amount is taken from your skyprivate.com account - so you don't need to do or pay anything before or after.

To understand better, take a look at this video here: