lots of different ways
1. iframe
2. modal dialog that, when launched, starts the payment process via a SYNCHRONOUS xhr connection to your/paypal server (and dismisses itself after completing - knowing that it can only complete after the transaction is finished)
3. use one of the paypal services that sends the browser to paypal completely and passes a linkback url for paypal to deploy once the transaction is complete.
the key thing to remember in the UI design is trust. the user must not have even a tiny hint that you might be spoofing paypal so everything must be and look completely above board. the more you go non-standard, the less trust you have.