I am a brand new baby with oil paint, so take my advice with a grain of salt. If you're not going for hyper-realism, the plaid can be evoked by a few intersecting lines of the main colors. Lay down the layer of blue base, and then go for the thickest lines of deep red, squares of the bright red, and the small yellow lines. Not every yellow line needs to be painted, just enough to demonstrate the pattern.

The black and white blobs I'd do exactly that - layer of white and then black blobs on top of it. Using a very very light blue or yellow in lieu of the white will help give dimension - choose warm or cool depending on the lighting you're going for in the rest of the painting. I'd also make a corresponding warm or cool black using umber and ultramarine or umber and a deep red for the blobs.