I love arm balances and Eka Pada Galavasana is definitely one of my favorites. It is also amongst the more challenging arm balances, and it requires some preparation or Kramas to master the pose. I will give you one way to prepare well for this beautiful arm balance, but there are always many paths to reach the same goal:) 

First, let’s look at Eka Pada Galavasana. It is a deep hip opening (meaning externally rotating) arm balance. In terms of joint movements we see deep flexion, abduction and a deep external rotation at about 90⁰ of one hip while the other is neutral or should be but sometimes stays a bit flexed - no biggie. Front knee is flexed, back knee extended. Spine is slightly flexed. Strong core and strong arms and shoulders are important. 
So, in order to prepare for Galavasana, we need to do a nice hip opening sequence to create space in the hip and, off course wake up the core as you’ll need it to lift up that back leg. 

If I do a deep hip opening practice, I love to start off with a Hip opening progressive namaskar. I usually do 5 A’s with different variations in Adho Mukha Svanasana and 5 Hip opening namaskars. But, any Surya Namaskars you usually practice will get your joints nice and juicy. 

In the standing  sequence make sure you use Virabhadrasna B, reverse Virabhadra, Parsvakonasana, and Ardha Chandrasana. They all prepare the hip for that 90⁰ rotation. I usually start off with different high and low lunge variations (side lunge is great), Virabhadrasana A and variations and slowly build towards Virabhadra B family. 

In seated sequence add asanas like Malasana, Baddha Konasana, Agnistambhasana (dbl pigeon). I would also suggest Supta Kapotsana – but stay high up to loosen up the hip flexors. You will need them cooperating to lift the back leg up. 
Do a core sequence you like and add an arm balance at the end, one that is not too challenging for you. Bakasana and Bhujapidasana are a good choice. And now you are ready for Galavasana. 

Before you try Galavasana I suggest immediate Kramas below. It's a sequence so you would hold each pose for five breaths.

To get in the pose - place your hands on the ground shoulder distance apart and lift your booty up keeping the foot over the knee. Lower down a little and hook your foot on your upper arm - this is a very important part. If the foot unhooks it will slide down. Your shin close to the knee should be on the other arm. Lean forward and see if you can lift the back foot off the ground. If you can, extend the back leg. Et voilà - Galavasana! 

In joy!