To begin, we've laid down the foundation. As a small courtesy, everything apart from the skirt has already been painted. First we need to roughly decide in our minds what kind of plaid pattern we want to draw.

Then we put in the base lines using the polygon tool. This time let's say we're doing a plaid pattern with sets of two lines, of which around six sets are visible overall.

Paint the area selected in step ②. Cancel the selection and move on to the next set.

With this, we have completed the vertical base lines. Next we'll paint the horizontal base lines in the same way as steps ②-④.

Horizontal lines completed. Now we're going to add the detail lines.

I tried adding thin white lines. Here we emphasize the realism a little by adding a black border using the boundary line in the layer effects (center). Basically, if you fill in the space between the base lines more closely, it looks more detailed.

Next, that empty space looks a bit bare, so let's add a white line in the space between one base and another. Since it's a red plaid skirt, green would work too.

To emphasize the areas where the vertical and horizontal base lines overlap, let's add a very dark green.

We add the shadows, and it's done.