Here's a link from Fulton Armory on how to do it.  Note the sub-link to the description of mechanically zeroing the windage.  Do this first, and follow instructions to the letter. HTH.

www.fulton-armory.com/tea/sighting.htm


Almost everybody leaves out an important step which applies to M1s and M1As, that of initial mechanical zeroing of the rear sight for windage.

You start by centering the rear sight (see pic in sub-link above).  Next step is to adjust windage by moving the front sight only.
Loosen the allen screw in the front sight and move it until you are centered for windage.  The rifle now has maximum amount of rear sight windage adjustment in both directions.  You will also be able to easily return to initial zero conditions if the rear sight windage adjustment has been changed.  Just line up the rear sight and receiver centerlines
I suggest you zero the rifle at 25 yards, and then confirm at longer range you should be close, if not right on.



Here's another link on how to zero your M1 with one shot:www.fredsm14stocks.com/column.asp?ITEM=20
The link to Fred's deals specifically with zeroing to get the appropriate Battle Sight Zero (BSZ).