Yep voice commands and Tapping behind her knee with the dressage whip. I use the word "Lift". I started just standing (as it should be and then moving to the walk. Now I do my work on the left side and am working on getting one leg going properly before I get both established. Put a bridle on them to start them off. Hold the reins up near the bit on the left hand side and raise them a little. Tap the dressage whip on the back of the leg behind the knee while using your voice commands. Praise when they lift their leg up (and bent) to start off with. Do it a few more times with heaps of praise when they lift. Quiet for the day on a good note. Next day, do the same but this time get the horse to lift its leg out instead of up. Don't praise them if they paw. They have to keep their leg there. As soon as you get the lift and extension, praise with feed and repeat a few more times and then quit for the day. If the horse paws it's leg impatiently keep tapping its leg and using voice commands until they extend and hold, even for a few seconds. Don't praise with feed if they pull their leg back under them.
It takes a while but I got Tess to this level in 2 days. Just consistently!
I hope I have explained it well enough (im so bad lol) and it works! When you are ready for the walking part, just ask. This part is still a WIP with Tess! Lol
She can hold her leg there for ages until I ask her to lower it. I can do it at liberty as well and in a VERY open paddock.