Wiki explaining installation here.
However it seems to discourage upgrading to Python 3 in PlugBox. This shouldn't be a problem since you can install 2.x alongside 3.x just fine.
According to post here the following should work fine if you are running Python 3.
pacman -S python2 python-distribute easy_install-2.7 pip pip install flexget
Note: There will be some GCC compiler complaints about optional speedups. Ignore these, those will fall back to pure python.
To verify installation: