One technique is here: http://www.outwaterphotogallery.com/video
Another Glowforge user had a novel way of lining up letters for a large wall sign. His method may not help you at this point since he used command strips rather than stand offs: First Post - Wall Quote via Puzzle Pieces
Finally, you can cut your letters out of paper/cardboard and use those templates to mark placement of holes in the wall that match where the standoffs are located in the acrylic.