Firstly, you need to enter your payment information in the following box:

Enter your invoice # (if you do not have one, please request us), your project name (given in the project proposal - does not need to match exactly), the amount due in the invoice, your first name and last name.
Once you have entered the above information, please ensure that the information is correct. When you are done, hit the "Submit Payment" button. You will be directed to the PayPal payment processing page.
Next, go on to Step 2: Follow instructions in PayPal.