What is the best way to create plaid on a cake? I would like to make a graduation cake using the same plaid as the school uniforms.

Glad that you ask this question. I've been wanting to do plaid on a cake.
Anyone?

If you have an airbrush, you can make stencils and spray the different stripes. Check out my tutorial on an even plaid at:
http://home-and-garden.webshots.com/album/570194886qYatNu
Click on the photos and read the explaination below the photo for more information.

I made a skateboard cake for a co-worker's son a few months back (see my photos) and wanted to get the plaid exact, so I had my co-worker take a photo of it and I made an edible image of it at Sobey's. I investigated all the options and that one was the simplest, cheapest and most precise.
There was a chocolate transfer option too, but that was expensive to order...There is also the painting fondant option (Aine2) has a tutorial, I believe.... good luck!

Thanks for that information.

I did plaid with an edible image that I had walmart print out for me. I just took them a pic of a print I wanted. It is on the ribbon around the cake in my avatar.

If you want to do plaid on fondant, it is quite simple. I used edible markers and a ruler on the tablecloth on the animal party cake and used a brush with dilted food colors in the plaid bird cookies. Buttercream may be a little trickier.

If you don't have an air brush use the cans of spray food coloring and a stencil cut from plastic, I use transparency sheets available at most stationary stores such as Staples.