Download the code and the unpack it (on some systems you may need to replace "curl -O" with "wget"):
  curl -O http://fastjet.fr/repo/fastjet-3.4.2.tar.gz
  tar zxvf fastjet-3.4.2.tar.gz
  cd fastjet-3.4.2/
Compile and install (you can also choose your own preferred prefix), and when you're done go back to the original directory
  ./configure --prefix=$PWD/../fastjet-install
  make
  make check
  make install
  cd ..
Now paste the following piece of code into a file called short-example.cc 
#include "fastjet/ClusterSequence.hh"
#include <iostream>
using namespace fastjet;
using namespace std;

int main () {
  vector<PseudoJet> particles;
  // an event with three particles:   px    py  pz      E
  particles.push_back( PseudoJet(   99.0,  0.1,  0, 100.0) ); 
  particles.push_back( PseudoJet(    4.0, -0.1,  0,   5.0) ); 
  particles.push_back( PseudoJet(  -99.0,    0,  0,  99.0) );

  // choose a jet definition
  double R = 0.7;
  JetDefinition jet_def(antikt_algorithm, R);

  // run the clustering, extract the jets
  ClusterSequence cs(particles, jet_def);
  vector<PseudoJet> jets = sorted_by_pt(cs.inclusive_jets());

  // print out some infos
  cout << "Clustering with " << jet_def.description() << endl;

  // print the jets
  cout <<   "        pt y phi" << endl;
  for (unsigned i = 0; i < jets.size(); i++) {
    cout << "jet " << i << ": "<< jets[i].pt() << " " 
                   << jets[i].rap() << " " << jets[i].phi() << endl;
    vector<PseudoJet> constituents = jets[i].constituents();
    for (unsigned j = 0; j < constituents.size(); j++) {
      cout << "    constituent " << j << "'s pt: " << constituents[j].pt()
           << endl;
    }
  }
}
Then compile and run it with 
  g++ short-example.cc -o short-example \
      `fastjet-install/bin/fastjet-config --cxxflags --libs --plugins`
  ./short-example
The output will consist of a banner, followed by the lines 
Clustering with Longitudinally invariant anti-kt algorithm with R = 0.7 
and E scheme recombination
        pt y phi
jet 0: 103 0 0
    constituent 0's pt: 99.0001
    constituent 1's pt: 4.00125
jet 1: 99 0 3.14159
    constituent 0's pt: 99