Its just 3 steps:
1. cd /etc/sysconfig/network-scripts
2.
cat <<~ > ifcfg-eth0:1
DEVICE=eth0:1
BOOTPROTO=static
ONBOOT=yes
IPADDR=10.1.1.2
NETMASK=255.255.255.0
~
3.
ifup ifcfg-eth0:1

(where 10.1.1.2 is additional IP address)