Place the skateboard upside down on a covered surface to allow your Shih Tzu to review it at his own pace.
Praise him and enourage him when he shows any interest in the skateboard. Spend a couple of sessions doing this before slowly turning the wheels to give him a chance to get settled before continuing to the next step. At this point, you should keep the skateboard upside down.
Once you've done this, turn the skateboard upright so the wheel are on the ground. Make sure to keep it at a reasonable distance so as not to scare your dog. Never move it straight toward your pup as this may startle him.
Move the skateboard outside, and continue to praise your dog when he touches or shows interest in the board. As your pup beomes more comfortable, you can begin to encourage your Shih Tzu to get on top.
Your pup may require some support, so you may have to put him on top of the skateboard. When he's on top of the board, whether he climed it himself or was lifted onto it, make sure to praise him.
As your dog becomes used to being on the skateboard, you can begin to slowly push him. As he feels more secure, you can begin to move him forward and backward, all the while praising your pup and rewarding him with treats.