I decided to tackle the tartan pattern on Slaine. He's typically wearing on in the pictures I've seen. Although the pattern isn't always the same, it tends to involve brown stripes on an orange background. These colors are consistent with an ancient tartan color scheme (red becomes orange, black becomes brown). I did a little searching and found the pattern below that I thought looked good. It hits the main colors and has enough going on to be interesting.
Painting a tartan seems really tough, but if you break the pattern down it's surprisingly simple. If you can paint stripes then you can paint a tartan. I've posted a detailed write up in the Gokan/Slaine tutorial on this website. The short version is you break down the pattern and start with the largest stripe. Paint the horizontal and vertical stripes, trying to keep the spacing consistent and follow the contours of the cloth. Then, where the stripes cross, go in and add the dark square. With those finished, pick the next most significant stripes in the pattern. Add in the color variations wherever those stripes cross over other colors. And then the next stripes, and the next, etc, going from the biggest stripes to the smallest.