I’m ROM flasher lover. I like to flash ROM as I think custom ROM is always better than stock ROM. In order to not losing any of my data in the phone, I always make backup before configure and flash ROM to my smartphone as most of the time you need to erase all the data and setting in the phone. Contact, Gmail and Google data are a must to backup as you need all these basic data for first time turn on after ROM flashing. This is a simple tutorial to show you how to backup all these important data without app needed.
You actually will be requested to sync all these information when you first setup your android phone. You may not remember it. Anyhow, you can do this configuration again after any ROM flashing.
1. Goto Settings- -> Personal => Back up and Reset
2. Check the Back up my data & Automatic Restore. Remember the backup account email.
3. Your contact, gmail and google data will be backup in Google Gmail account in cloud.
Example above is using my Galaxy S3, you may have slightly different UI on your smartphone but should work flawlessly and identical.