For the plaid, I would use a plaid stencil
culinarystencils.com
You simply place the stencil on the fondant and paint. First stencil is vertical, then horizontal. Of course you can do it with stripes you do by hand, but plaid looks much better done with a very crisp edge. Use coloring that is a bit diluted (not solid) It's the second stripe that makes the pattern come alive as it deepens where it covers the first set of stripes.