Note: As each version of iOS makes slight variations to the layout and options in iOS, this information should be used as general guidelines as your options and configuration may vary slightly.
From the Settings app, go to the Wi-Fi section and click on the network name
A pop-up will open automatically, the look of the screen may vary depending on the phone vendor
In the Username field enter your username
In the Password field enter your password
On the next screen, you can see the details of the certificate. Click Trust on the top right corner to confirm
The phone will attempt to authenticate on the network
Once the network data has been stored in the device, you will need only step 1 to connect again (you just have to click the network name in the Wi-Fi section).