To start using QUID, simply shop at any seller that uses QUID and click the pay button. No downloads, no apps, no friction. After you've paid once, you only need to sign in to make further payments.
QUID lets you make purchases directly on your credit card. For more about this, see How do I get billed?
You can also sign up for a QUID account from the QUID website. User accounts are free. You only pay for what you purchase.