Flashing CM 11 on Verizon Samsung Galaxy S3 (SCH-I535) I have a Galaxy SCH-I535 from verizon (SGS3). It is not rooted, and I want to install CM 11 on it. 

Is there anything else I have to do besides just running the installer?

I love my phone and don't want to break it. Do I have to root or install anything else before running the CM11 installer app? And, what should I use to back it up before hand in case anything goes wrong?

Could someone tell me what I should do step-by-step before actually running the CM 11 installer?
Too lazy to post a detailed answer. But no, you need don't necessarily need root to install a custom ROM like CyanogenMod. But, you need to flash a custom recovery like ClockworkMod (CWM) recovery which you will use to install CyanogenMod. Some custom ROMs (like CM) will give you root.

For detailed installation guide for your phone, refer to (CAUTION: This is for CM 10.2, but the steps should be similar for CM 11) CM's official wiki . It contains all the information you need, including back-up.