Bridges can be a simple but effective activity if done correctly. The biggest mistake is not keeping the spine stable while moving the hips.
A good way to keep the spine stable is to pull one thigh to your chest and hold it there while you bridge up, this helps avoid a lot of spine movement. You can progress a bridge by moving your hand and upper body positions, which forces you to activate the core a little more to maintain core and spine stability while moving the hips.
You can also work more hamstring flexibility by straightening the knee of the flexed hip while bridging up. Always start at the most fundamental or basic level until you are proficient before moving on to more difficult positions, always remember quality trumps quantity.