Put a soft medball under your low back and tuck your feet back under the bench like a bench press set up. Lay back over the ball until you get a good stretch in your hip flexors. Then flex your abs to sit back up. Everyone is different so experiment with the ball placement until you find the one gives you the beat stretch
Start out using just body weight. Do 4 sets of fatigue and build your way up to 4x25, once that becomes easy then you can start adding weight.