This is probably the most difficult procedures for repairing the Magic Mouse. The reason isn't so much because it is difficult technically. The Ribbon cable just slides in to the attaching point. The problem is it is in a odd position with little room to hold the cable. You will need a small thin set of needle nose pliers or hemostats. Attach the hemostats to the ribbon cable near the middle of the cable (enough to hold the cable stiff. Align the cable with the slot and try to push the cable in. Once it is in be careful not to pull it out again. Put the mouse back together and it should work fine. Follow the instructions in ifixit Magic mouse tear down procedure.