Under /etc/network/interfaces scroll down until you find your network interface card in the file (usually named eth0 for an ethernet connection, or wlan0 or wifi0 for a wifi connection).
Change 'iface eth0 inet dhcp' to 'iface eth0 inet static'
Add the following lines, substituting the IP address numbers with your desired configuration:
As they are probably additional IP addresses, you will use for example
if eth0 is your current interface
iface eth0:1 inet static
iface eth0:2 inet static
Change the IP and netmask accordingly. You don't need to specify the gateway for additonal ip's and the broadcast and network will be configured for you anyway, it's not necessary to add those, but you can if you want. You also want the auto eth0:x lines as well, so they come up automatically when restarting the network/rebooting etc.