I've dealt with this a bunch. there is a hole that the spring goes into and then you place the rubber piece back over it. There are two holes in the arm of the brake setup and which one you use determines the swing amount of the brakes.
it might be easier to take out the brake cable completely when re-assembling.
when re-inserting the brake cable back into arm and tightening down, be sure not to tighten too hard- these nuts and bolts wear out fast. one of mine is actaually rigged with custom nuts and washers cause i stripped the stock parts
it gets easier the more you tinker