By "Bridge" do you mean, like a plank, where you hold the top position of a push-up (sometimes done on the forearms, rather than the hands)?
If so, the first step would be simply to hold the bridge or plank position. You want to keep a straight line from your shoulders to your ankles. By putting your hand o an elevated surface (like a bench or step) you're decreasing the resistance, and making it easier. I would shoot for time, try to work up to 2 "sets" of about 30 seconds.
From there, it's just a matter of raising either leg, not super-high. The higher the lifting, and the more you move your leg around, the harder it will be. Hope that helps some.