1) Opening the hips is key to single behind the leg pose. This pose opens the low back, chest and knees! A solid way of working towards this is learning the splits, baddha konasana and half lotus forward bend. I've made several youtube videos that focus on this, one of which is called 'Opening to the splits'. Click the link to watch it.
I also feel that half lotus is a requirement first because of the pressure on the knee. There are a lot of great ways to prepare for this, which does not come from bending your knee into the posture. I personally did a lot of work on my legs and hips before lotus. Here's one exercise that also strengthens your stomach. This posture is called supta padangushtasna (it's great for your legs). Working on your legs will help lengthen the muscles and gives you more space to make the rotation in the hips when the time is ready for lotus or called padmasana. See the link above.
2) Arm strength. Believe it or not the hardest part is opening the hips. A lot of this pose is more about balance and leverage. However, solid planks, arm pressure pose and other arm strengthening poses will help you a lot. Some postures that will indirectly give you the power and the confidence are headstand and variations as well as crow pose.
3) Once you have the leg behind your head and feel comfortable you can move to the next stage, which brings the posture together. Practice just to lift your pelvis upward and hold. Once you can hold for 5 breathings, you can easily shift the pelvis forward while bringing the leg toward you.
This video shows you this transition including a more advanced way of entering another arm postures called astavakrasana
It takes time, patience and practice. I am a believer that many people can learn to do this, but again opening the hips can be a a few years. I myself worked for 5-6 years on lotus, forward bends and even twists to open the outer hips! Failing any injuries or knee issues when your back opens the leg goes too! The rest is practice and finding your balance. It's kind of like when the student is ready the master appears.
More instructional videos on basic postures of the AtmaVikasa system can be found on my youtube channel.