The 12 or 13 volt power is too low to break thru the resistance of your dry skin so there shouldn't be a problem even if you touch both terminals at the same time.
The red lead goes to the positive (+) terminal. The black lead goes to the minus (-) terminal.
Connect the red + wire first.
This doesn't have anything to do with the direction the electricity travels.
It has to do with preventing a short circuit.
The entire frame and engine of the motorcycle is connected to the black (-) negative wire.
If it is connected to the battery first, almost every exposed metal part will be connected to the battery.
If you then attach the red positive (+) wire you will see no reaction.
If you then try to tighten the positive wire and your wrench or screw driver happens to touch any other metal part of the bike it will instantly cause a short along with sparks, loud popping and you filling your pants.
I know of a guy who attached the negative ground wire first and then started to attach the positive wire using a wrench. He was wearing a ring on the hand that held the wrench.
As the wrench turned, his ring contacted a metal part of the frame. The power went from the battery thru the wrench, thru the ring and into the car frame.
The ring almost melted.
Anyway, always attach the red (+) wire first. Then when you are attaching the black (-) wire, even if your wrench touches the frame or engine there will be no possibility of causing a short. After all, the black wire is already making contact with the frame and engine.