what brand, most maufactors websites will show wiring diagrams
here is a simple one
In a 12/24V system, the most efficient is three wires coming to the trolling motor. You will have a common ground for both 12 and 24V. Then you will have a pos wire coming from the battery the ground is connected to will be the "12V" wire. Another wire from the second battery pos will be the "24V" wire. A jumper at the battery between the neg of the second battery and pos of the first battery will complete the 24V circuit. Hope this helps.