Normally I would first install the clear screen protector, but I already have a Zagg Invisible Shield on the front and back of my iPhone and don't want to remove it, thus rendering it useless, therefore I will skip this step. I can say that my past experiences with OtterBox screen protectors has been less than stellar as they are very difficult to install perfectly and you only get one shot. My suggestions would be to just take your time and work in a dirt / dust free environment.
Installing the phone in the Commuter case is an easy process. First you must fit the phone inside the silicon jacket, then simply slide the outer shell over the jacket. I would suggest inserting the top part of the phone in first, then work on the bottom. I found it best to slide the port flap through the shell first before pushing the rest of the phone in. It may take a minute or two to get a perfect fit; just keep working with it and it will eventually fit just fine.
Before we move ahead to usage and a conclusion, let's check out the Defender series.