I am a HUGE monty roberts plan. I would really suggest getting his book "The Man Who Listens To Horses" It goes through how to do join up and is full of information (obviously). They had all Monty Roberts books at my Library so I would suggest seeing if there's a copy there.
You can do join up in any size arena or with no arena at all, but a round pen is ideal. If you are going to use hay bales, maybe put them two high so the horse doesn't hop out of the arena.
To do join up, you will want a long lunge line, your round pen or join up area, and if you are going to try and saddle the mini, a saddle, snaffle bridle, and pad. You could also use a surcingle if you prefer.
Bring the horse into the arena and send him away. To send the horse away, you are going to square up to the horse and lock your eye on his. This sends him the message to move away. Once he starts to go around the arena, you are going to continue to remain square and keep your eye on his. You can use your lunge line to throw at his hip and get him continuing forward. After a few laps the horse is going to begin to negotiate with you. I usually lope a few rotations in either direction then begin to see the horse communicate. the first thing you look for is his ear locked onto you, focusing on you. The next thing you're going to see is licking and chewing. He's beginning to say that he wants to come back to you and have a chance to join your "herd".
The last thing you're going to see is the horse will drop his head and his chin will be bobbing along on the ground as he moves. This is the final step. Youre now going to move your body to a fourty five degree angle to the horse and drop your eyes off the horse.
The horse at this point will stop moving away and will come up behind you and touch your shoulder with his muzzle. If he does not want to join you, get him moving until he shows the signs and try again.
Now move off and make a circle in each direction, when the horse follows you, this is follow up. :)
His description in the book is by far better, but there's a basic outline.