You can perform Cisco 1941 & Cisco 3945 configuration using the following steps:
ip flow-export destination <NFA Server IP> 9996
ip flow-export source {any l3 interface} //loopback interface of the device
ip flow-export version 5
ip flow-cache timeout active 1
ip flow-cache timeout inactive 15
Enable "ip flow ingress" command on all the layer 3 interfaces.