How can I configure netflow on Juniper?

Juniper supports flow exports by the routing engine sampling packet headers and aggregating them into flows. Packet sampling is done by defining a firewall filter to accept and sample all traffic, applying that rule to the interface and then configuring the sampling forwarding option.

interfaces {
    ge-0/1/0 {
        unit 0 {
            family inet {
                filter {
                    input all;
                    output all;
                }
                address 192.168.1.1/24;
            }
        }
    }
}

firewall {
    filter all {
        term all {
            then {
                sample;
                accept;
            }
        }
    }
}

forwarding-options {
    sampling {
        input {
            family inet {
                rate 100;
            }
        }
        output {
            cflowd 192.168.1.100 {
                port 2000;
                version 5;
            }
        }
    }
}

Other options exist such as aggregated flows, which are detailed at: http://www.juniper.net/.