Here are a few quick tips -

1.You should choose a pumpkin that is flat on one side (so that it doesn't roll)

2. The eyeball gourds should match in color and size as much as possible

2a. The eyeball gourds should have stems that are long and sturdy enough to shove into your pumpkin. (You can cut the eyeball holes with a knife or a screwdriver. Make the hole just smaller than the gourd's stem for a secure fit)

2b. The eyeball gourds shouldn't be just one color. It should have a darker "iris" to make it look more like an eyeball. Check out these examples

(you'll notice the dark one on the right doesn't look as good as the others. (It's best if you can see the "whites" of their eyes)

3. then fill in the best mouth groove with a black marker. And cut off the stem.