We assume that you have already logged into Zen Cart.
Click on the Modules link found on the main menu as shown below.

Then click on Payment link, like the one below.

Zen Cart allows you to easily integrate several payment methods in your store.
With the module highlighted, click to install it.

Then click on the edit button to edit the payment module.

Authorize.net is used when you wish to allow real time online credit card processing with your own merchant account.
Type in your login Username, Transaction Key, Transaction mode (be sure to set to production when you’re ready to go live), Transaction method,

You have an option to set a value for order status.

You can set order to display this option to your customers, the lower the number the higher the display.

Click on update button.

That’s it, Authorize.net has been successfully set up.
Installing the credit card  module allows you to collect credit card details online and manually process the orders offline.
Let us try to install the PayPal module.

First highlight the PayPal module and click on the install button besides it.

Sort order of display to “2”

Then click on the update button.

You might also find this tutorial useful.
That’s it, you now know how to add payment modules in Zen Cart.