Fear is the biggest problem here, we're never quite sure how far our feet are away from the mat behind us.
Overcoming the fear.
Knowing our body will be comfortable in that position helps. Before dropping into Viparita dandasana from headstand, try it from the mat first. Push up into udhva dandasana then bring the hands to the head, cupping it as in headstand with your forearms and elbows on the mat. Finally walk the feet out into viparita dandasana proper.
From viparita dandasana, walk the feet back in and hop up and down a few times. your not trying to come up but just to get a feeling for how you'll drop into the position.
Walk further so you have to go up onto your toes, hop up and down again this time on your toes again just to get a feeling for how you'll land on your toes.
Notice how as you walk in your chest comes forward, this will be key for dropping out of headstand but also for hopping back up again.
If you still lack confidence consider putting some large firm cushions at the end of the mat to drop on to.
You might start with several cushions and then take one away each time until your finally dropping directly onto the mat.
Lowering the legs into urdhava dandasana (pic.2)
As you lower your legs, push your hips backwards as far as possible
Keep the lower back flat, don't allow it to round
Stretch out through the legs
If you push your hips back far enough they should act partly as a counter weight to your legs allowing your legs to stay horizontal without too much effort.
The drop (pic. 4 & 5)
As you take the legs over you need to counteract the weight of the legs as much as possible giving you time to lower them, with control, as close to the mat as possible.
Push down into the mat with your elbows, this is your anchor
Drop your shoulder blades down your back
Inhaling push out your chest, bringing it towards your chin, imaging your pushing tour chest out and down towards the mat.
Allow your back to arch
Tuck your tailbone and engage the bandhas as fully as possible
While doing all of the above, lower the legs further and further behind you you.
Your knees will be bent, allow your feet to drop to the mat landing on your toes and then your heels.
Once landed, take a breath and walk your feet out as far as possible stretching through your torso and legs.
Coming back up (pic. 7&8 )
Walk back in to the point at which you landed when you dropped.
You want to bring as much weight forward as possible
Drop the shoulder blades down your back, inhaling push your chest out and down towards the mat, arch the back as much as possible.
When your ready exhale, hold the exhalation and hop up
It's the hips that will carry you over, bend your knees and when you hop up think about lifting your hips rather than your feet and legs.
As you hop up push your hips up and forward over your shoulders and your chest down towards the mat.
You want to catch the point just before you were forced to drop, you don't have to hop all the way up just to that point and catch the point of balance.
Once caught, draw your legs closer to your body and careful release the arching of the back raising the hips, the knees and finally the feet