Instead of stepping out with your front foot so that your knee is at a 90-degree angle, take a slightly larger step. This way, when you lean forward you'll have increased tension on your hip flexors. If you feel pain instead of mild tension, though, reduce your step a bit. It's important to step out further instead of trying to lean further forward with your foot in the same position. During your stretch, your front knee should never extend past the toes of your front foot. As you lean forward on your left leg, for example, raise your right arm straight up in the air. Then, angle it inward over your head very slightly. Stretching your arm upward will add further tension to your hip flexors. Extend your arm upward, but don't stretch it up so far that you lose your body positioning for the stretch. Keep your back straight, your shoulders slightly back and down, your chest slightly out, and your abs contracted. For the alternative stretch, start by lying flat on your back on your exercise mat or towel. Use both hands to pull your left knee up to your chestcome as close to touching your chest as possible without causing pain. Then, extend your right foot so that your right calf is flat on the mat (or is as close as possible without pain). This will mildly stretch your right side but not your left. Make sure to stretch both of your legs evenly. Hold this position for 10-30 seconds, and do 3-4 total stretches per leg.

Summary:
Step your foot out further if you don't feel mild tension. Raise your opposite hand as another way to increase the intensity. Do an alternate hip flexor stretch if this one is too challenging.