Yes, that is the correct approach.
Be sure to get a MONO 3.5mm cable (not a stereo 3.5mm cable). If you already have 3.5mm cables, you should be able to inspect the tip and see if it has 2 connection points then it is MONO. If more than 2, then it is stereo.
So, when you cut/strip the end that will go to the screws, you should have 2 wires in there. The way that I would handle figuring out the +/- wire would be to plug the 3.5mm end into the AVR sending the signal, and clip my Volt Meter to the bare wires on the other end. Then, send out the 12V trigger signal. If the volt meter reads +12V then you know the negative lead of the meter is connected to the negative wire for the 12V trigger. If instead you get -12V, then you know that the negative lead of the volt meter is connected to the positive wire of the trigger.
Obviously, you will need to configure the 12V trigger in the AVR menus first.
Something like this. You don't care what is on the other end, you are cutting it off.
https://www.parts-express.com/parts-express-35mm-mono-extension-cable-6-ft--240-004