Found it very helpful during this step of reassembly to first separate the two halves (top and bottom) of the JoyCon case by disconnecting the two ribbon cables attached to the charging rail. This allowed much more space to work with when reconnecting the ZR button’s ribbon cable, working from the side for better sight lines and tweezer insertion. Just needed to reattach the two charging rail ribbon cables thereafter, rejoining the two sides of the case, which was very simple.