What Terminal Do You Connect First On A Battery. Ensure that the connections on both ends are secure by trying to move the battery around. Any mistake with the other lead will lead to a short.

Then after unhooking the negative terminal and setting it aside disconnect the positive terminal. Properly connect the terminals will help the factory marking determining the polarity applied to them. Here is the order.
What Happens if You Disconnect The Positive Terminal First.
You really do not want this. And you can accidentally hit that body with the positive lead. Tighten the terminal with a wrench. And car batteries are very good at shorts.

