I had some help from my coach who has taught probably 100 horses to do this. But by the end of half an hour I was parking her myself...granted she still needs to work on standing still during a walk around if I am not holding her...but the basics are there.
Here is how I did it.
First off, if you plan to show your horse in halter, walk them around the arena at a show speed (might as well add that in so that they remember that in the ring) then stop somewhere in the middle (during training the first few times stop in the same spot as they remember the spot and what they did), stand in front of your horse (my coach said this means stop, always, no negotiation and you make sure they know that (that way in the ring you stand I front of them and they stay put)) next...back them up so that they back feet are aligned.
Once the back feet are square, stand beside the horse, (this is where it took me the most time)
First: this is for the leg furthest from you and the action results in lifting the weight off of the leg you are trying to move: hold the lead on the halter and pull forward while with your other hand you are pulling the withers towards you then push with your back foot to get them to move that foot forward (pulling forward on the halter will aid in them recognizing what you want)
Next for the foot closest to you: again pull the halter forward while with your other hand you push on the withers while pushing forward with your toe on the foot closest to you.
Then use a treat or whatever to start and get them to hold their head high with their ears forward and give them the treat up there.
I spent half an hour on this and by the end of it I would put slight pressure on her withers and slight pressure on her feet to move them forwards and she was starting to lift her head without a treat being there.
I have a ways to go, but she had never been parked before and I'd never parked a horse ewither and she picked it up within a half an hour, so the method is effective!
If you're looking to mount easier I have seen horses who do a form of bow to allow their owners on, not the same as a park, but maybe that is something you want to try for you guy?