For start - configure flow templates

vitalvas@mx204-city17# show services flow-monitoring version-ipfix
template ipfix-ipv4 {
    flow-active-timeout 10;
    flow-inactive-timeout 10;
    template-refresh-rate {
        packets 2000;
        seconds 30;
    }
    option-refresh-rate {
        packets 8000;
        seconds 120;
    }
    ipv4-template;
}
template ipfix-ipv6 {
    flow-active-timeout 10;
    flow-inactive-timeout 10;
    template-refresh-rate {
        packets 2000;
        seconds 30;
    }
    option-refresh-rate {
        packets 8000;
        seconds 120;
    }
    ipv6-template;
}

Configure instance for export

vitalvas@mx204-city17# show forwarding-options sampling instance ipfix-inline
input {
    rate 1000;
}
family inet {
    output {
        flow-server 192.0.2.33 {
            port 2055;
            autonomous-system-type origin;
            version-ipfix {
                template {
                    ipfix-ipv4;
                }
            }
        }
        inline-jflow {
            source-address 192.0.2.1;
        }
    }
}
family inet6 {
    output {
        flow-server 192.0.2.33 {
            port 2055;
            autonomous-system-type origin;
            version-ipfix {
                template {
                    ipfix-ipv6;
                }
            }
        }
        inline-jflow {
            source-address 192.0.2.1;
        }
    }
}

And enable on chassis

set chassis fpc 0 sampling-instance ipfix-inline
set chassis fpc 0 inline-services report-zero-oif-gw-on-discard
set chassis fpc 0 inline-services flex-flow-sizing

After you can configure sampling on interface or another place