1. Your entire doll doesn't have to be finished like mine, but the garment you'd like to add the leopard print to should be shaded. (It doesn't necessarily have to be pixel-shaded either, but I find that it's easier.) Mine is on a base from The Doll on the Hill Factory.

2. This was written for khaki-ish colors, as I was going for a sheer effect. I've given you a couple of palettes to experiment with. If you want something more dramatic, experiment with color combinations.

3. Create a new layer. Draw small spots with medium gray centers and light grayish-brown edges, or vice-versa (which is more accurate) if you prefer. This can be time-consuming, so if you're lazy like me, copy-and-paste is your friend.

4. Shade the print according to your garment's shading. Your shading can be as subtle or intense as you want...experiment and see what looks best to you.

5. This is optional, but I think it can make the pattern pop. Select the Burn tool, set it to a soft round brush (I think I used the Soft Round 5 pixel brush) and around 20% exposure, and go over the pattern where there should be shadow.
Then switch over to the Dodge tool (I set mine to Soft Round 7 pixels and 10% exposure), and use it where the light would hit. Be careful, though—dodge too much and your pattern might disappear.

6. If you like, play with the opacity of the leopard-print layer. I set mine to 90%.

(Please do not adopt the first two examples)
Here are a few things I've made using this method. Evangeline's pattern (featured on the far right doll) was created with the gray outside and brown inside.