From the bottom of a lunge, extend the front leg and push it forward as far as your current flexibility allows.
Once you get close to your lowest position, stand upright and do not use your hands for support.
Alternate between relaxing in this position and engaging your legs by pushing them into the ground to strengthen the stretched muscles.
Form tips
Keep the hips "square": do not let them open to the side (letting the position become a mix of front split and middle split). Keeping them square will intensify the stretch on the back leg's hip flexors.
The back foot can have either toes extended, or toes curled in. Toes curled in might help squaring the hips, as it tends to make misalignments more visible.