The sonar, lights and engine starter need to be run on 12 volts.
If you take a meter to your boat, you'd find that measuring from the negative of the first battery to the positive of the second (in other words, put your meter on the same posts the trolling motor leads are on), you'll have 24 volts. Now, if you put the meter on both the positive and negative of the same battery (doesn't matter which one), you'll have 12 volts, even though the jumper is still connected.
What these guys are saying is that if you put the other leads on the positive and negative of the 'same' battery, you'll still have 12 volts. That said, it's strongly preferable to put it on the first battery in the circuit (battery that trolling motor negative is attached to - think of the power starting there, then gaining 12 more volts as it runs through the second battery - definitely oversimplified, but you get the point). Reason for using that battery is that if somehow the negatives got crossed, and your sonar ended up tied with the trolling motor negative, it still would lead to the bottom battery, and wouldn't give your sonar 24 volts. Were you pulling the 12v off the second battery, crossing negatives would end up feeding 24 volts. Chances of negatives getting crossed are next to none (this is more used in cases with common negatives - such as a car - a shared negative is still workable, even with two voltages), but this is still good practice.
So, to answer your question, connect the sonar/light/engine negative lead to the same post that the TM negative is on, and the sonar/light/engine positive lead to the positive post of the 'same battery.'
Dawson