After you are able to perform 3 sets of the short bridge, the next step towards the full bridge is the Straight Bridge. This exercise activates your spinal muscles as well, but this time you gonna need arm and leg power (mostly arm power!) to perform it, while short bridges just require you to activate the spinal muscles mainly by pushing through the legs. Due to the fact that you need both, arm and leg power, to perform a full bridge later on, this exercise is the logical second step. Straight bridges are a very hard exercise for your triceps and the muscles between your shoulder blades, too. So if you want to perform full bridges one day, you should be able to perform straight bridges first. For me this exercise is pretty though. My feet always slip backwards slowly while I perform the exercise so I suggest that you search for an anti-slip floor when you start performing it.
Let's start with the how-to-perform explanation:
Sit down with your legs straight in front of you. Your knees should be totally straight and your feet should be approx. shoulder width apart. Place your palms on the floor (fingers in the direction of your feet) on either side of your hips (or a bit backwards). Now press down through the hands, tensing the arms as you simultaneously push your hips upwards by using the spine muscles until your body forms a straight line. Draw the chin up and look towards the ceiling. Your bodyweight will be carried through your palms and heels at that point. This part of the movement should last 2 seconds. Hold the pose for 1 second and reverse the motion until you almost touch the ground again (2 sec). Now start the next rep without a rest. If you can't manage to meet the target instantly, do every set as much reps as possible until you can do the requested amount.
Target: 3 sets of 40 reps
Paul recommends to do the exercise only once a week but I prefer to do it twice (see "The Plan -Part II").
If you can't manage this exercise properly (like me in the beginning) you can ease it up by doing it with bent legs (like in the short bridge) first until you reach a level of strength where you can perform it like it should be.