It is very convenient to cross tie your pony since it keeps your pony centered and allows easy access to both sides your pony. Prior to teaching your pony to cross tie, it is imperative that he be able to stand quietly for an extended period (e.g. 15 minutes or more) of time. That is, the pony should not be fidgeting, pulling, pawing, etc. when tied. It is good to use verbal commands such as "whoa" to indicate to your pony that you wish for him to stand still.
A crosstie consists of two ropes attached to facing posts or walls. The end of each rope has a snap, usually a quick-release clip, which fasten to the pony's bit or halter on either side of the pony's face. Your pony is now held in place by the ropes attached to his halter. The ropes are usually just long enough to touch each other, thus, when the pony is tied, there will be enough slack for a little shifting and movement, but the pony will have to keep his head up, so it is important to watch your pony while tied in case he slips. For this reason, it is always good to have quick release snaps. The photo on the right shows a bio-equine in cross ties while he is being tacked up (notice that the cross ties do not have panic straps where they attach to the bio-horse's halter).
Make sure you have a safe place to tie your pony. Use non-slip flooring, such as rubber mats or dirt since slick surfaces, such as concrete, can be dangerous especially if your pony is wearing hoof boots or other shoes with elevated heels.
The ropes should be cotton or nylon (leather can also be used if you like). You could you rubber or elastic materials, but be careful if you choose to do so. Make the ropes just long enough so that when attached to the posts or walls, the crosstie snaps will barely touch each other in the middle. This will reduce the risk of your pony getting tangled up or turned around when tied.
Note: You really should not tie your pony to anything by the bit. That being said, I enjoy being tied by the bit in many of my scenes, and I personally believe that while this is not safe, neither is crossing the street. Thus, I do not advocate this practice, but you should not be leaving a pony unattended regardless of how your pony is tied. Just be safe.
Now that you have cross ties setup in a safe area with good footing, lead your pony over to the cross-ties and have him stop. Once your pony is standing nicely attach both cross tie ropes (via their snap clips) to his bit or halter and detach the lead rope/reins, or alternatively rest them around your pony's neck.
Let your pony stand in the cross ties for a couple minutes while you walk around and pet or brush him. This will help calm the pony and let him know the cross ties are safe and he can relax. Reattach the lead rope or reins (if you detached them) and release pony from the cross ties and put him back in his stall. Do this a couple more times (if possible on different days) leaving the pony (make sure you stay nearby for safety) in the cross ties longer each time until he fine standing still in the cross ties for 15 - 30 minutes.
Now you can groom and tack up your pony in the cross ties without having to worry about him moving around.