Place your battery in the rear panel, with the cable side down, and use two small strips of electrical tape to hold the cable against the solder pads.

Place a strip of tape over the end of the battery ribbon cable to both cover two of the solder pads and hold the contacts down against the logic board.

Heat the rightmost contact until the solder below melts and apply a small amount of solder to the contact, allowing it to flow through the two holes in the cable and down to the logic board.

As soon as the solder has flowed into the joint, remove both the solder and the iron.