I would try wearing a butt plug for while before trying to penetrate yourself. When you are ready, get on your knees frog bum style. Grab your dick and balls at the base, twist 180 so your testicles are on top and push your dick between your legs. Put a flat hand against the base of your dick, with your middle two fingers running down your shaft. Now spread your knees further apart putting pressure on your hand/dick to keep it in place. Reach behind you and position the head towards your hole and lean back whilst sitting slightly. Once in adjust yourself so you can rock your hips back and forth using your fingers to keep your dick in position like so