Make sure you have iClarified added as an installer source. You can find directions on how to do that here.
Press to launch Installer from your Springboard.
Press to select the Install tab at the bottom of the screen.
Press to select iClarified from the list of Categories.
Press to select Volume Boost (1.1.3) from the list of Packages.
Press the Install button at the top right hand corner of the screen.
Press the large red Install button that appears.
You will get a Notice telling you that this process will patch your iPhone to increase its maximum volume! Press the large OK button.
When the installation is done you will get a Notice telling you that your volume has been patched and to restart your iPhone. Press the large OK button.
You will be returned to the Categories list. Hold down the Sleep/Wake button for 3 seconds to bring up the Power slider. Slide it to the right to turn off your iPhone. Then push the Sleep/Wake button again to turn back on the phone!
You will now notice an increase in the volume of your phone :)