add 1 battery,connect black lead to neg of one battery,connect red lead to 2nd battery +, make a jumper wire with equal guage wire to open positive and open neg posts. you have 24 volts at trolling motor. your red wire needs 50 amp breaker or fuse near battery.
the explanation Dazz provided is good, ill add that if its a Motorguide, when purchasing a new motor, if memory serves me right, the jumper you will need comes in the package, if not, i likely have a handfull of them in the cabinet in the garage from all the new motors ive had, or installed for others over the years