Using your standard mushroom carving knife, cut out your pattern. It's recommended you place foil at the bottom of the pumpkin and under the top of the lid to reflect light and thoroughly illuminate your design. Be sure to insulate your leds if you place them at the bottom to prevent them from shorting out.
To increase the spread of light throughout the pumpkin, I sprayed each led with glass frosting spray paint. If you don't want to drive to Home Depot and spend $4 on that, you can always sand them with low grit sand paper (wear eye protection, i've heard of them exploding before)
I used 5 leds in each red, green, and blue. They are all 5,000 or 6,000 mcd (brightness). I then soldered them together and hooked them up to two 9 volt batteries. Only the red LEDs needed a lot of resistance (they are only 2v). Then, be sure to insulate the wires before getting them near aluminum foil.
Put foil in the bottom of the pumpkin. Put the leds inside. (if you want you can place a toggle switch on the outside of the pumpkin) Then, put it on your front porch and show off your Super Mario Pride!