Go to converter, see + wire going to battery
Look at - wire. Negative the other wire that goes to the battery.
Look at the top of the capacitor it has two phillips head screws.
One is marked +
The other is marked -
Think you can undo one nut on the converter and hook + to + ?
Then undo the other nut on the converter and hook - to - ?
Means buying a length of twin wire, two #10 X blue insulator ring terminals
And two bigger hole ring terminals for the converter studs.
Heck you can even connect this direct to the battery if you want to.
This is pretty basic and the capacitor makes a great ripple suppressor.
Some LED's flicker when they see a certain amount of NOISE in the 12 volt supply. You proved that yourself.
Fixing a flat on a bicycle tire is 100 times more complex than this capacitor and twin lead install.