You can manually create a file and add the rules, but I recommend you to use the 'nmcli' command. It's new in Red Hat 7 / CentOS 7.
The 'nmcli' command automatically updates the files, and it makes changes persistent across reboots.
# man nmcli-examples # nmcli con add help
After you finish making changes run:
# nmcli con reload # nmcli con down <interface name> # nmcli con up <interface name>