Go to CRM Menu Merchants > Processors. click on "+ New", select Authorize.net from the processor list within the Credit Card tab. Create an API login and transaction key within Authorize.net and use those as the Username and Password for the processor setup. Fill in the remaining fields for Processor Name (you can name this anything), Merchant ID, Descriptor (optional), Monthly limits, and extra optional options. Make sure you select "No" under the Test option for live merchant accounts. Once complete, you can begin testing transactions.