pip install flexget
or (from a normal user's shell)
sudo pip install flexget
Note: For Archlinux users pip refers to python3-pip. Use pip2 to install flexget as pip2 refers to python2-pip.
This will install FlexGet and all additional components it requires.
If you get pip: command not found pip was not installed correctly in previous step.
Starting from this step, you shouldn't use root account for anything.