How do I do it?

The boat has two 12v batteries connected in series for a 24v trolling motor. Assume the motor's 12v starter battery is dead and I'm on the lake miles away from the ramp.

Hooking up jumper cables, do I disconnect the trolling motor batteries or is it safe to leave them connected and hook the jumpers up to +/- terminals of ONE of the batteries?

Yes you can jump off one battery without unhooking the other. I carry a jumper box with me seems to work better that jumper cables for me anyway.

Jumper boxes are better if you have the room in your boat. Jumping off your TM batteries can be risky if you have been running them all day.

Before "jumping off"any TM battery turn off all power,
AND disconnect TM from the Marinco plug.
Consider the B&D 500 amp Jump Starter as an option.

Don't leave the booster cables connected. Guy did this and his stainless prop looked like someone had been welding on it. Blue arcs and streaks all over the prop.

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spare prop?

all you have to do is hook up one battery to the other, for safety leave the hatch open for a couple of minutes to let any fumes escape, battery or gas fumes can accumulate in the bilge area and are explosive.

don't have to unhook anything, I have jumped my start battery a few times, jumped others a few times as well and never a problem.

always hook up pos to pos first, ground of tm battery next then grnd of start battery.

When jumping the cranking battery to a trolling motor battery, also be sure to select the trolling motor battery that has the trolling motor's negative ground connected to it. (Some boats already have that TM b battery ground strapped to the cranking battery ground.) Give the cranking battery a few moments to accept some charge from the trolling motor battery before hitting the start switch.