My favorite Christmas song by far. For the Am6 it goes like this

Am6
--1--
--1--
--2--
--2--
--0--
You don't have to play it you can just skipit and go staight into the D at the beggining 
"Christmas".

G                 Am   (Am6) D
I'm dreaming of a white Christmas
C             D             G    D7
Just like the ones I used to know
          G         C9
Where the tree-tops glisten
    C        Cm
And children listen
   G    C9     G            D7     D
To hear sleigh-bells in the snow

G                 Am    (Am6) D
I'm dreaming of a white Christmas
C          D                G    D7
With every Christmas card I write
         G       C9    C   Cm
May your days be merry and bright
        G        C       D      G
And may all your Christmases be white

And thats it, you can keep repeating it as many times as you like I find it best to 
a D7 at the end for a turnaround if I'm going to continue. Merry Christmas