It's very easy. When you've found a product that you'd like to buy, simply click on "Add to Bag". The product will then be added to your bag. If you are signed in, it will remain in your bag until you've completed the checkout process or until you remove it. If you're not signed in, it will remain in your bag until you close your browser.
To checkout, click on "Place Order" and you can then pay safely through PayPal with either your Visa, MasterCard, American Express, Discover or with PayPal funds.
You don't need to have a PayPal account in order to pay through PayPal.