The Philly shell defense posture is not for beginning boxers. It's a riskier defense that sacrifices some coverage for the ability to counterpunch more swiftly; however, successful fighters like James Toney and Floyd Mayweather Jr. used it with devastating effects during their careers.
While you're learning, expect to get your bell rung because of the limited coverage. With consistent training, you'll reach the point where it gives you an advantage in the ring.
In the Philly shell stance, the fighter positions his lead arm across his torso with the tip of his leading elbow forward. Then he rests his other hand on or slightly below his chin.
To practice the Philly shell, get into your standard boxing stance: feet about hip-width apart, back straight but ready to flex. Set your lead hand across your torso, running at rib or abdomen height with your fist on the opposite side underneath your elbow. Raise your rear hand to the side of your face. Hunch your lead shoulder up toward your ear and tuck your chin to protect your face as much as possible.
Hold the position tight as you use shoulder rolling, blocking and ducking techniques to avoid your opponent's blows. Move the rear hand from side to side to protect the face. Lean back slightly to deflect overhand or straight punches.
Use the elbows of both arms for defense, pointing them out at your opponent. Roll the right and left shoulders forward, opening the elbows out to the sides for blocking. Slip to the sides of incoming punches and respond by using your rear hand to throw punches.
The Philly shell requires significant mobility if you want to keep your head safe. Shoulder rolls are a good place to start for loosening up the shoulders. Simply roll one shoulder forward, then roll the other shoulder forward. Alternate 20 to 30 times.
Practice moving in the stance before you attempt to use it. Hold the position tight as you move your rear hand around the face to protect it. Roll the shoulders forward without losing the arm position. Thrust the elbows out in front of you, and open them out to the sides in a shoulder roll. Simply performing these movements will increase mobility.
Train your dodging skills by using a slip bag. A slip bag is a lighter bag that swings on a chain for you to dodge. This is one of the best tools for building the reaction time you need to successfully use a Philly shell.
Work with a sparring partner once you can dodge the slip bag more often than you don't. Practicing the Philly shell in the ring against an opponent motivated to punch you in the head is the only way to improve your shell enough to use it in a live fight.