If you don't already have a QUID account, then simply pay with your credit card on any merchant that supports QUID. When you make your first payment, a QUID account will automatically be created for you. No downloads, no apps, no friction.
If already have a QUID account, simply hit the pay button on any website that uses QUID, and sign into QUID (if required).
You can also signup for a QUID account from the QUID home page.