Bring it to someone who know to solde...
Be very very carefull in STEP 23 it isn't a Philips #00 screw. It thiner and If you try with another screw you will get a widther hole and you will not able to mount the SIM reject part. So I advise you to keep this little screw separetly and remember to use it when you mount your iPhone again.
How do you identify is a battery of 1150mAh?