There are useful shorthands like quote, but ultimately macros are transformations from AST to AST. For your example:

# INPUT
iex(1)> quote do: x > y 
{
  :>,
  [context: Elixir, import: Kernel],
  [
    {:x, [if_undefined: :apply], Elixir},
    {:y, [if_undefined: :apply], Elixir}
  ]
}
# OUTPUT
iex(2)> quote do: data.x > data.y
{
  :>,
  [context: Elixir, import: Kernel],
  [
    {
      {
        :.,
        [],
        [
          {:data, [if_undefined: :apply], Elixir},
          :x
        ]
      },
      [no_parens: true],
      []
    },
    {
      {
       :.,
       [],
       [
         {:data, [if_undefined: :apply], Elixir},
         :y
       ]
     },
     [no_parens: true],
    []
   }
 ]
}

The underlying pattern looks like:

{:some_var, _, _} -> {{:., [], {:data, [], Elixir}, :some_var}

You’d apply a transformation like this using a function like Macro.prewalk or your own recursive traversal code.

Once you’ve got the data-fied expression, you’d wrap it in the AST corresponding to fn data -> ... end. You can find that shape using quote:

iex(4)> quote do: fn data -> :ok end
{:fn, [], [{:->, [], [[{:data, [if_undefined: :apply], Elixir}], :ok]}]}