I like what your trying to achieve it seems like fun! This looks really tough, and I've have not even tried to do plaid. But here are some ideas.Cover any baked and cooled cake with a smooth layer of fondant. Hold a thick- or thin-stripe stencil over the cake and spray with a mist of cake coloring spray. Have fun and hope it turns out great!!!
I am thinking that you use a black edible ink pen and of course use a ruler to get them straight but I would think that the fondant would have to set up first so that the pen wouldn't dig in the fondant.
You could also easily iron on a plaid chocolate transfer sheet. It's nice because you can stiil smooth the fondant.
http://www.americanchocolatedesigns.com/ has several different plaids.
You could also easily iron on a plaid chocolate transfer sheet. It's nice because you can stiil smooth the fondant.
http://www.americanchocolatedesigns.com/ has several different plaids.
Please tell me more about ironing on a plaid chocolate transfer sheet 
I've done a couple of plaid cakes. Both were piped buttercream, with the lower levels of lines smoothed gently with a Viva paper towel. It's not difficult to do, but looking back at these now, they could have been neater. But here they are as an example:
http://www.sugarfrosted.com/displayimage.php?pos=-846
http://www.sugarfrosted.com/displayimage.php?pos=-1490
I tried to get a chocolate transfer but it would have cost $60 because I'm in Canada. So, I went with the edible image. I also used the edible marker idea to do the plaid on the panda- turned out well. Thanks for all the great suggestions!Here's the finished product... 
It looks really really nice, I think it's great!!! In fact I am adding it to my favorites.
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