After added products on your cart, click "My cart" on the top right of the screen to make sure what you want to buy are on the list of your cart.
Click "Checkout" then move to Checkout page.
You can find Pay Pal on the top left.
(sorry for all images below are Japanese because pay pal shows only Japanese to me. Anyway, follow red line in images.)
Fill in your email address or phone number here(red line) and click "next"(blue bar below red line in image) to move to password page.
Enter the passwords(red line) and click "log in" (blue bar below red line in image) to log in.
Please make sure your information(name,address etc,,) is correct and chose credit cards.
Click continue(Blue bar on the bottom of the form)
After you chose payment method,Click"Continue to payment" to move to payment page.
Make sure Shipping address, Shipping method, Billing address are correct.
Do not forget to make sure Pay Pal is checked
It is recommended that check the "Remember me" to faster check out nextime.
Click "Complete order" to complete your order.