To use window tags, import this module into your ~/.xmonad/xmonad.hs:

 import XMonad.Actions.TagWindows
 import XMonad.Prompt    -- to use tagPrompt

and add keybindings such as the following:

   , ((modMask x,                 xK_f  ), withFocused (addTag "abc"))
   , ((modMask x .|. controlMask, xK_f  ), withFocused (delTag "abc"))
   , ((modMask x .|. shiftMask,   xK_f  ), withTaggedGlobal "abc" sink)
   , ((modMask x,                 xK_d  ), withTaggedP "abc" (shiftWin "2"))
   , ((modMask x .|. shiftMask,   xK_d  ), withTaggedGlobalP "abc" shiftHere)
   , ((modMask x .|. controlMask, xK_d  ), focusUpTaggedGlobal "abc")
   , ((modMask x,                 xK_g  ), tagPrompt defaultXPConfig (\s -> withFocused (addTag s)))
   , ((modMask x .|. controlMask, xK_g  ), tagDelPrompt defaultXPConfig)
   , ((modMask x .|. shiftMask,   xK_g  ), tagPrompt defaultXPConfig (\s -> withTaggedGlobal s float))
   , ((modWinMask,                xK_g  ), tagPrompt defaultXPConfig (\s -> withTaggedP s (shiftWin "2")))
   , ((modWinMask .|. shiftMask,  xK_g  ), tagPrompt defaultXPConfig (\s -> withTaggedGlobalP s shiftHere))
   , ((modWinMask .|. controlMask, xK_g ), tagPrompt defaultXPConfig (\s -> focusUpTaggedGlobal s))

NOTE: Tags are saved as space separated strings and split with unwords. Thus if you add a tag "a b" the window will have the tags "a" and "b" but not "a b".

For detailed instructions on editing your key bindings, see XMonad.Doc.Extending#Editing_key_bindings.