So many dancers have great flexibility when stretching, but find it hard to use this when dancing, especially with getting legs higher in a développé. If you simply practice the step over and over, often the result is bulky thighs and tight hips, with not a lot of increase in range. This will happen if you are trying to use the front of the leg to lift, rather than controlling the leg from the hip.
Slowly peel the foot up into a retire position (feel your inner thighs working!)
Make sure to keep the foot flexed if you have any issues at the back of the ankle (i.e. Achilles Tendinopathy or Os Trigonum Pain)
Make sure that the hips stay square, and don't hitch or twist!
Slowly unfold the leg as for a développé devant at 90 degrees working on controlling the turnout from the hip, and lengthening the leg.
Focus on not cheating height by tucking the pelvis in this position, but focus instead on correct placement of the leg
Slowly rotate the working leg in and out in order to master control of the hip in the socket
Slowly lower the leg back to the floor, making sure you use your abdominals so that your back doesn't arch!
Repeat just a couple of times on each leg
If you can control the leg and pelvis well with a développé to 90 degrees (or above) without moving the hips, you will be able to achieve a much greater height of leg when you let the pelvis become involved (it has to begin to shift a little above this). However, if you are hitching and struggling to get to 90 degrees, you will find it very hard to improve beyond this point unless you really focus on your stability!
If you practice your développé in this position, your muscles will learn how to control the leg much easier, and you will find that your range improves quite quickly! Then, when you practice standing, everything feels a lot more familiar!