When monitoring network traffic, a flow measurement approach can provide the advantage of data reduction in near real-time. RTFM, the IETF's standard, generalised architecture for measuring traffic flows, and NeTraMet, an open-source implementation of that architecture, are introduced. The results of a NeTraMet usage survey are presented, together with a detailed description of the NeTraMet components and the ways they can be used to construct traffic flow measurement systems for any particular network. Nifty, an X Window near-real-time traffic flow analyser, is also described. Experiences with NeTraMet are summarised, highlighting its usefulness in building customised flow measurement system.