Log into each cam, individually, with a browser (likely IE) using the cam's current IP address.
Go to network settings and insure that the camera's IP is set to 'static', not 'DHCP' or 'automatic'.
Ideally, assign a unique IP to each cam that is outside of your router's DHCP pool (the IP's it uses to assign to DHCP devices in your LAN). For instance, if the router's DHCP pool is 192.168.1.2 to 192.168.1.99, use 192.168.1.200 to .254 for your devices that you want to have a static IP.
Log into the router's LAN IP with a browser to discover what its DHCP pool is. While there you can either make a reservation for those static IP's =OR=
just use IP's outside of the router's DHCP pool as I suggested above (easier, IMO).
If the router provides that ability, of course. Unfortunately, many ISP modem/router combos don't, so using IP's outside of the router's DHCP pool will provide the same result in that the router won't attempt to assign your chosen IP('s) to another device(s).