Just to share my diy experience after changing two iphone batteries
1. buy brand battery from official store e.g Yoobao, others probably wont last long
2. just follow ifix instruction, and get a better "suction", those free with battery kit isn't strong enough. make use of hair dryer to heat up
3. hardest part is opening the screen (as its glue is strong), removing battery (use alcohol and card to scrap away the battery glue)
4. patient, first timer probably need 1 to 2 hours to finish the job or else just pay others to do for you