Please help creating this iRule to direct subnets to different paths.

There's no specific syntax, you just have to define a datagroup with all your subnets in it. You can create it through the Web interface or via tmsh.

Here is an example of what you have to define :

ltm data-group internal /Common/Network {
records {
    DMZ {
        data 192.168.10.0/24
    }
    LAN {
        data 192.168.0.0/24
    }
}
type string
}