Summary: that it attaches to, very carefully peel up the contact strips from the ends to break the welds. Try to leave the whole strip in one piece and solder the ends back on the new cells. Buried in the very middle of the pack is a thermistor used by Roomba to sense battery temperature. This is an important device and must be reinstalled. A dab of hot glue stuck it back down.
Image Dimensions: 460 x 345
Image originally found here.