How do you hook up a motorcycle battery charger?

You should follow the instructions supplied in the owner's manual or instruction sheet that came with the charger unit.

If you did not receive a manual or sheet, ask at the store where you bought the charger. They may photocopy another one if you ask politely.

Contact the manufacturer of the charger and ask for one.

Check out the Support or Manuals page of the manufacturer's website to download a manual or information sheet.

Generally you charge a battery in a well ventilated area as explosive gases may be given off. You connect the battery to the charger BEFORE plugging the charger into the mains outlet. You should set a timer to alert you when the charging cycle is complete. It is usually recommended that you do not smoke or use naked flames near a charging or charged battery. Naked flames include storm lanterns. Also take care when switching on an electric light. The switch contacts may arc briefly generating a spark that could ignite in an explosive atmosphere. Persons following this answer do so at their own risk. I accept no responsibility if you are unfamiliar with the safe use and safe operation of your equipment.