Sandhills Bass Anglers, North Carolina

One thing that I did is add a minnkota alternator charger. It charges the trolling motor any time I start the big motor. It is a fairly inexpensive option and easy to install. May help your issue.

To clarify your question.  You currently have a cranking battery for the big motor and a battery for your trolling motor.  You are wanting to know how to wire in another battery in series so you have 12v still but it will last longer? 
Or do you have one battery for cranking and trolling motor and you are wanting to add a seperate battery for your trolling motor?
If you can tell me what you have now and then exactly what you are trying to do, I can help

 I know I have extra cables that run to t/m. I have motor guide 46 lb t/m with 3 prong plug. Know one is positive and the other is negative. The neg wire is loose and not sure how to hook up another battery because of that one cable. Don't want to fry my t/m.

I removed my other reply because I misread the comments.

If you have two batteries that you are trying to wire to your trolling motor while maintaining 12v you need to wire them in parrallel.  Hook a jumper from POS to POS and then a jumper from NEG to NEG.  Then power supply to your TM will be wires from trolling motor to NEG and POS on the same battery. 
This will give you the SAME AMPS AND THE SAME VOLTAGE.
If you just add a jumper from one battery to the other you will be supplying 24v to your TM, thus smoking it

Pic on its way Mike.

I guess I'll just have to get a new battery, some wing nuts and change batteries on the fly. Just hate running out of power after 6 hours. Of course that's mainly on windy days. After I get my taxes back, I'll get the alternator charger.

If you purchase another battery for your TM, wire it in parallel like I described earlier.  Your voltage will remain 12v but your amp hours will double.  If you have a couple decent deep cycle batteries hooked in parallel, your trolling motor life on one charge will be tremendous. 
No need to add more electrical components.  One thing I have learned after years of messing around on boats is the simpler you can have your electrical, the better.  Not only is it easier and cheaper right off, it is easier to maintain, inspect, less potential for failure and cheaper to replace should something go bad.
again, wiring two batteries in parallel:  you need two 2' lengths of 6awg wire (autozone, one red, one black for easier ID).  Use one jumper (red) to go from POS on Batt 1 to POS on Batt 2, the other jumper (black)  from NEG on Batt 1 to NEG on Batt 2.  Then from POS and NEG on Batt 2 are your 12v supply to the TM.  Another good thing to add to ALL connections on a boat is dielectric grease.  Keeps moisture/corrosion off connections.
Also even though they are of common use, wing nuts have a reputation for loosening themselves giving you the "pucker factor" when you are in the middle of the lake (or offshore 24 miles) and your motor doesn't start.  Then you realize after 30 minutes of frantically running around the boat that the wing nuts came loose, again.

Won't use wing nuts on crank battery. Just extra t/m battery. That way I can switch on fly if I need to. I'll make sure to also have lock washers. No loose connections for me. I hate that. Only thing I don't like about alternator charger ( that I read ), is that they charge t/m battery after crank battery is charged. With electronics on and short runs, not sure it that would work for t/m battery.

Exactly.  They work good for a boat that the main cranking battery is ONLY used for cranking battery and nothing else.  As soon as you power fishfinder, GPS, livewell, bilge, lights etc... that is alot of amp draw and the battery charger will probably spend all it's time charging cranking battery. 
Your best bet might be to, when you get your other battery, wire them in parallel for your TM.  That'd give you up to 14 hours of trolling time.  Then, when you get home just plug in your charger.