To use, first import this module into your ~/.xmonad/xmonad.hs file:

 import qualified XMonad.Actions.FlexibleResize as Flex

Then add an appropriate mouse binding:

     , ((modMask x, button3), (\w -> focus w >> Flex.mouseResizeWindow w))

For detailed instructions on editing your mouse bindings, see XMonad.Doc.Extending#Editing_mouse_bindings.