don't suppose by any chance you're lucky enough to have a double lunge arena?
Then, second question, is your horse prone to kicking and do you have someone who can help you?
Since I didnt have anyone who could help me, I used our double lunge arena. Put the two lunge lines on my mare and threaded them through the rings of a lunging surcingle. Then, she knows the command 'walk on' so I said that and then pushed her bum (I know she doesn't kick, people anyway) and stopped once she moved forward. She was very confused in the beginning as she's used to lunging but luckily, with the double lunge, all we had to focus on was walking forwards with me behind her which took a little while. Now we have to work on the turning part...
But if you don't have one, and you have someone to help you, then stand behind your horse, slightly to one side for safety sake, and get someone to stand at your horses head. Then give the command, and get the person to lead the horse forward until your horse is used to you being behind him/her with him/her walking forwards. Then I found it useful to have your assistant stay for a little while at your horse's head without actually leading him/her.
That's all the advice I have to offer, I've still got a lot of learning to do about ground training myself.