Open the .p12 or .pfx certificate file using your iPhone by selecting the file. This will start the process to install the certificate as a new profile on your iPhone. In the example below, the *.p12 file was included in an email.
At the Install Profile screen (shown below) press the Install button.
You will be prompted with a warning message that says, "Installing this profile will change settings on your iPhone." Press the Install Now button.
If your device has a PIN or passcode set, you may need to enter this in. After that (or if no passcode has been set), then you will be required to enter in the password for the certificate. Using the keyboard provided type in the password into the field. Once you have entered in the password push the Next button at the top beside Enter Password.
The certificate will continue to install. Once it has finished you will be presented with the following screen.
You can exit this screen by pressing the Done button in the top right hand corner.