Important to remember that your foot is in front of your knee for this stretch to do anything. You must push down in the centre of your body and lean back slightly.
Keep your front leg straight and reach forwards worth your arms aiming for your nose to touch your shins. You'll feel this one on the backs of your leg, also flex your toes to progress this stretch.
Use a step with a cushion under your ankle. Best practice is with arms raised as it pushes your groin towards the floor but you can put hands either side of your front knee in the floor.
The important thing about full splits is not letting your hips twist. Start from lunge position as in fig 1 and gradually straighten your front leg.
To progress or maintain a stretch such as splits you need to practice regularly. I stretch daily for about 30 minutes doing various stretches. But realistically aim for a session a few times a week, and when you're sitting around watching TV. Warm up first, e.g running up the stairs. Hold each stretch for 10 seconds, do this 3 times and push yourself a little bit further each time.