1. Once you are sure you are using a supported browser and that you are operating from within one of the supported countries, you will need to acquire Live and Sandbox API keys from Authorize.net
For sandbox API keys, log into the sandbox Merchant Interface. For Live API keys, log into the Live Merchant Interface.
Click Here for more details on Sandbox vs. Live Mode in Authorize.Net.
2. Then you can proceed to Payments> Integrations>Drop the 3 API Keys you got from your Merchant Interface in Authorize.Net.
You can input the Live API Keys in the Live fields.
And you can input the Sandbox API Keys in the Sandbox Fields.
Once inputting the desired API Keys, click on the Save Button.
3. It will show you a quick prompt asking whether you want to make Authorize.Net your Default Payment Gateway.
Clicking on Cancel will still integrate Authorize.Net but will not make it the Default Gateway.
Clicking on Confirm will integrate Authorize.net and make it the Default Payment Gateway.