You can use this module with the following in your ~/.xmonad/xmonad.hs:
Then edit your layoutHook and add the Master modifier to the layout that
mastered (1/100) (1/2) $ Grid
This will use the left half of your screen for a master window and let
Grid manage the right half.
For more detailed instructions on editing the layoutHook see
Like XMonad.Layout.Tall, withMaster supports the
XMonad.Layout.Shrink and XMonad.Layout.Expand' messages.