in the cehckout wizard, you would need to select PayPal Payments pro, then delete the checkout form on the cehckout page.
you _could_ delete the checkout button on the cart page, copy the checkout with payapl button from the checkout page to the cart page, and copy the code for the authenticate transaction with paypal express checkout server behvaor from the checkout page to the cart page as well to bypass the checkout page. I would not recommend doing this unless you are familiar with and comfortable with editing PHP code, though.