1. How do I use this for payment? 

 First, you need to discuss commission with me. Then I tell you a price for your exact wish. After we agree - you need to pay fixed amount as down payment, in the "down payment" post. Now you wait for sketch. If you love everything and wish me to proceed - you paying rest of the price in "goals".
So, fixed pre-payment before sketch, that is not returnable, and then rest of the price, that was discussed.

2. How many times I can change something in the process?

 You have unlimited fixes on sketch phase, on rendering phase it's 3 small fixes for free.