when you are in your forward splits - lets use the left leg in front for an example. place your hands on the floor, right next to your hips. Slowly shift your weight to your right hand. As you're doing this, rotate your hips to the right as well. You may need to use your hands to gently lift yourself off the ground, leaving your feet on the ground. Bring your left arm to the same side as your right as you are rotating. As you do this continue to rotate to the right. Eventually, you will find yourself in the straddle position. You may, depending on your hip flexors, need to bend your arms once they are on the same side. Lean on your forearms for a good stretch of the hip flexors. Hope this helps - also trying googling "split stretching".