Once you've soldered the battery flex on, make sure to insulate your solder points else they'll ground out on the case, which wouldn't be good. Also, as you can probably see electrical tape is a bit too stiff for this application, so Kapton Tape is much better if you have it (or can get it).

Unfortunately when I re-did my battery due to this one failing, I forgot to grab a photo showing how it's supposed to be done!