Here are the steps I used to carve an optical illusion pumpkin, but they can apply to almost any design you want to try!

Step 1: Create a Stencil

Once you decide what pumpkin you want to carve (in this case, I chose an optical illusion pumpkin based on a tattoo by the very talented Jesse Rix), create your stencil. You can do this either by drawing freehand, or tracing over the image. While tracing paper can be useful, I find that the quickest way to trace a stencil from an image is using an iPad (you can also use a lightbox if you have one, but most people don't and they don't come cheap).

Install the free app filmScannerLite, which acts as a lightbox by illuminating the screen completely. Place your printed image and blank paper on the iPad (I use a transparency sheet as a barrier so the pen doesn't bleed through to the iPad screen) and trace the image. If necessary, you can trace components separately and combine them into a stencil later using software such as Photoshop or Pixlr (which is free).

Tools:

Image(s)
Printed image
Blank paper
Transparency sheet
Pen
Tablet or lightbox

Step 2: Get Your Pumpkin Ready & Apply Stencil

Choose a pumpkin that has enough space for your stencil, and look for one that doesn't have too many ridges. The smoother the pumpkin, the easier it will be to carve! Wipe it off with a damp paper towel to clean off any dirt, and dry it before applying the stencil.

First, tape Saral transfer paper over the area you plan to carve. Saral paper is an easy way to get your stencil onto the pumpkin quickly and cleanly, and it's generally available in most art supply stores. Because the pumpkin is convex, you will have to fold creases into the paper to get it to lay as flat as possible - you may have to try a few times, but you'll get the hang of it!

Once you have your transfer paper on the pumpkin, tape your stencil on top of it. Again, you'll have to fold creases into the paper; try to make the folds where they don't interfere too much with the drawing. If that's not possible (for instance, if you're doing a face and you have to fold the stencil in a way that cuts an eye in half), you can always adjust your drawing later with a pen directly on the surface of the pumpkin.

With the Saral paper and stencil in place, trace over the lines with a ballpoint pen. Remove the paper and spray the pumpkin with fixatif (also easily found in an art supply store); this will keep the stencil from rubbing off while you carve. IMPORTANT: fixatif is a spray with pretty nasty fumes! I highly suggest spraying it outdoors, and leaving the pumpkin outside for 10-15 minutes afterwards to let the smell dissipate.

Tools:

Saral paper (I use white, though it comes in many colors)
Fixatif spray
Stencil
Tape
Ballpoint pen
Paper towels

Step 3: Get Your Tools Together

Now you have your stencil on the pumpkin and it's time to carve! The tools you use are entirely up to you, and I've added various tools to my kit over the years. However, I'd say the essentials are a linoleum cutter, large and small potter's loop tools, and a permanent marker. Other tools I use are an x-acto knife, wax carving tools (similar to dental tools), and various needles.

Step 4: Block Out Areas of Dark & Light

I find it helpful to start a pumpkin carving by blocking out the lightest and darkest areas. Use your permanent marker to shade in the darkest areas, and start using your linoleum cutter to peel a layer of skin from the lightest areas.

Step 5: Continue Carving!

With the darkest and lightest areas blocked out, continue shaving, peeling, carving, and scooping out the pumpkin layer by layer. The deeper you carve, the lighter that area will be.

Once you have a preliminary pass done on the entire design, it's time to hollow out your pumpkin! This is a crucial step, and if it's done well it will make the rest of your job MUCH easier.

I recommend cutting a hole in the back of the pumpkin, NOT around the stem. A pumpkin stem is full of nutrients, and leaving it in place will extend the life of your pumpkin by several days at least! Use a spoon and your hands to pull out the seeds and pulp. Once the inside of the pumpkin is relatively clean, use your large clay loop tool to start scraping out the meat of the pumpkin behind the stencil. Using the clay tool will be much quicker than using a spoon! I suggest scraping away at the pumpkin until it's just about 1/4 inch to 1/8 inch thick. The more you scoop out on the inside, the less you will have to carve on the outside.

Tools:

Knife (to cut open the pumpkin)
Clay loop tool
Optional: spoon
Optional: rubber gloves (to keep your hands clean!)

Step 6: Finish Carving in the Dark

Now it's time to refine your carving. Turn off the lights or go into a dark room, and put a light inside the pumpkin. Any light will work, though the brighter the better - if you have a battery-powered lantern, a flashlight, or even a phone with the flashlight turned on, that will work! You can put your light inside a ziploc bag to keep it from getting pumpkin slime on it.

IMPORTANT: Don't use a candle! It won't give enough light, and the inside of the pumpkin will burn.

With the pumpkin lit and the room darkened, you can really start to see how the pumpkin will look. Continue scraping away and adjusting until you have the levels of light and dark that you want. Remember, the deeper you carve, the more light will come through that spot.

Tools:

Carving tools from before
Flashlight, lantern, or phone
Ziploc bag (optional)

Step 7: Enjoy Your Creation!

When your pumpkin looks finished, step back, and enjoy the results of your hard work!

Unfortunately a pumpkin won't last forever, so don't forget to take pictures. Various websites have suggestions for prolonging the life of a pumpkin - I've heard about lemon juice, vinegar, vaseline, plastic wrap . . . however, none of those have worked well for me. I find that the best way to keep your pumpkin intact for a week or more is to keep it in the refrigerator when it's not being displayed. Otherwise, it will rot in a few days.

Happy halloween!