How to configure NSEL (~NetFlow) on Cisco Firepower Threat Defense (FTD) using the FlexConfig feature introduced in Firepower Management Center (FMC) software version 6.2
See the attached doc.
Note that in a few versions of FTD code, the Flexconfig deployment for NetFlow as given in this document, may fail. This is due to a minor bug. Check out my comment in this article (scroll towards the bottom of the page) talking about this bug and its workaround.
Note that this document is applicable only if you are using managing your firewall using FMC. If you are using the on-box management functionality using Firepower Device Manager (FDM), then you may want to look at this article
https://community.cisco.com/t5/security-documents/how-to-enable-netflow-on-firepower-using-fdm/ta-p/4048081