Click on the PayPal link on the merchant's website. Depending on how the website is designed, you may have to choose a "PayPal" option from a payment type selection screen, then click "Continue" or "Submit" before you can move on to the PayPal payment screen.
Look for a section on the payment screen that reads "Don't have a PayPal account?" You should see a link titled "continue checkout" somewhere in this section. Select this link to open the credit card input screen.
Enter your personal information, billing address and credit or debit card information into the appropriate lines on the input screen. Click the "Review Payment" button when you are done.
Review the payment details displayed on the screen to make sure everything is correct. If you need to edit any of the information, click the "Change" button at the bottom of that section. When you are satisfied that your credit or debit card information is accurate, click the "Pay Now" button to finalize your payment.