This is one of my favorite costumes, and I've used it many times.

Makeup:

The key to this costume is really the makeup, and the very best makeup to use is water-based makeup. Look for water-soluble face paints - some look like crayons that you dip in water, and some look like watercolors and are applied with a small paintbrush.

The best face paints I have ever tried are by "Klutz". If you can get your hands on them, buy some! They're easy to apply, they're non-toxic, and they rinse off very easily. However, if you have some other kind of makeup, you can use it as long as it comes in the necessary colors.

Below are some step-by-step instructions. I came up with them by looking at pictures of real leopards and copying the pattern. Trust me - it's incredibly easy and it looks amazing when it's finished.

1. Start by smearing some white over the mouth (like a moustache), and over each eye (like eye-shadow). Also, paint the chin white, but don't go much further than the edges of the mouth on either side. The sides of this white blob should be slightly rounded.
2. Now, take some mustard-yellow face paint (or the closest shade you can get to it) and paint most of the nose, cheeks and forehead as shown. Try to blend the white of the "moustache" into the yellow. You can leave the tip of the nose unpainted for now.
3. Paint the top of the forehead and the tops of the cheeks orange. Try to blend the yellow into the orange.
4. Take some black paint and carefully trace the area of each eye, as illustrated. (Try to follow the line of the eyebrow where possible.) Then, make some little dots over the mouth. Paint the lips black. Finally, draw the shape of a leopard's nose on the tip of the nose. Put a black line right down the middle of it, and continue the line all the way to the mouth. (If you don't believe me, look at a real leopard - that's what their noses look like!)
5. Now for the spots: Use black paint to make squiggly spots all over the face, letting them overlap some of the area above the eye. Note that the spots should be small and delicate near the center of the face, and gradually get bigger and bolder near the top and sides. Also, make squiggly rings and swirls rather than solid blobs.
6. Make some long black and white whiskers over the mouth. Add some little black and white whiskers around the eyebrows - a few near the inside and a few more near the outside, as shown. Put some white whiskers below the mouth, on either side of the chin. Next, take some shade of orange and put little blobs inside the black spots to bring them out more. Finally, fill in both sections of the outlined nose with some really dark orange.

Now start growling!

Accessories:

You can always use store bought leopard ears and a tail, but sometimes you can make much better ones yourself.

For ears, you can take a hair band and make construction paper ears to attach to it. If you can find some furry leopard patterned hair scrunchies, you can use them to form the ears. That's what I did - I bunched together two scrunchies to make each ear, and I used some wire to attach them to the hair band. By securing each "ear" tightly to the hair band at each side, and letting the middle of the "ear" stick up, I got a pretty realistic shape.

For a tail, I was lucky enough to find some furry leopard patterned fabric. I rolled it up and used some wire to make it keep its shape. You can also try using leopard print stockings if you can find some. You can let one leg of the stockings hang out as the tail, and scrunch up the rest of the stockings under your clothing and out of sight.

Clothing:

If you wear all black clothes, the makeup and accessories will be enough. However, you will look even better if you can find some leopard print clothes - women's leopard print tops are pretty common. Maybe you can borrow something from a friend if you don't have any of your own.

For an extra touch, if you have some exposed skin on your arms and/or legs, try painting on some big leopard spots - no background, just the black squiggles with some orange inside them.