For a basic fixed-amount set up, set the minimum payment value to exactly what you want users filling in the Form to Pay. For charitable donations and invoice payments, set the minimum payment to something minimal like $5 which avoids you making a loss on payment gateway fees for tiny transactions. Then look to Steps to Allow User to Decide Amount they Will Pay
In the Secure Zones tab, pick a Secure Zone. Access will be granted when payment is made and the Form is submitted.
Back in CMS Forms, find the ID for your newly created Form:
Use this ID in the Liquid tag when you insert the Form on the Page (or use Toolbox as a quick shortcut):
Your Payment Form is now set up and ready to accept payments using the minimum payment value you set.
If your Payment Gateway is in 'Test Mode', then you may need to enter card details specific to the testing environment.
If you are using the Stripe Payment Gateway, this step is optional.
For PayPal or Authorize.net, you will need to follow the relevant extra steps:
See SiteBuilder to install a plug and play, themed Form Layout for your payment Form:
And set your Form to use it with the layout parameter.
See the Forms Reference for creating your own Layout:
By default, the Form will charge the minimum amount, but for some use cases like donations or invoices, you may want to allow them to pay more: