Disconnect the negative terminal first. Then take the positive terminal off.

This removes the chassis from the electrical path, so you don't melt yourself in case a wrench touches the positive terminal and the chassis at the same time. (With the negative terminal, it doesn't much matter because it's already connected to the chassis through the ground wire - there's no voltage difference.)

Installation is the reverse (positive first, negative second). The negative terminal will still spark on contact (when reattaching) because things like clocks and stuff will be running, even with the ignition off, but it's not really a problem.