Lighting your pumpkin is simple. There are basically 3 options:
For candle power, the best bet is to use votive candles. These are short, and sit flat very well. If you have cut the bottom out of your pumpkin as recommended, then lighting is easy: Just sit your candle on the ground, light it, and place your pumpkin on top of the candle. Be sure if using a candle, to cut a small "chimney" at the top, back of the pumpkin to allow in oxygen.
For battery power, the most effective thing we’ve found for lighting your pumpkin is the new LED camping lights that can be found online. Usually found in a round "UFO" shape, these can be found that allow varying levels of lighting strength so you can adjust them to the level you need.
There are also some other battery powered pumpkins lights sold at Halloween, but try to stick with the LED types. Incandescent bulbs types will run the batteries down very quickly.
For electrical power, the preferred lighting product is a small, clip-in light sometimes used in doll houses or to light Christmas village houses. They can very readily be found at the $.99 store, and work quite well. You can either just sit them directly under your pumpkin, or cut a hole into the side and clip the bulb into it.
Another alternative is simply a string of small Christmas lights. These can also add a special "sparkle" to your pattern for an interesting effect.
Whenever using electrical lighting, always use the size bulbs for which the receptacle is rated. Always keep the original fuses and fuse holders intact, and always plug your extension cords into a GFCI receptacle. This will avoid the potential for electrical shock.