Step 1: Ingredients
You need to end up with 180g of each flavor of Skittle. Since Skittles are approximately 1g each, you can accomplish this with 180 skittles. We used two 450g bags for our experiment -- other bag sizes are perfectly acceptable.
(Of course, if you want to experiment with making the vodka to be more flavorful, you could definitely try more skittles than this. This was what we found as a good flavor level, without using too many bags of Skittles.)
Can you use other candy with this same method? We think it's very likely that this will work well for most fruity candy generally available (like hard candies or chewy fruit candies). We're not sure how gummy candies would work with this -- but it's definitely worth experimentation.
The vodka quality is actually pretty important. We found that using middle-quality vodka created an end-result that tasted a lot like medicine (particularly the cherry flavor). I would suggest trying a better-or-best-quality vodka instead. Several commenters have pointed out that you can try running middle-quality-vodka through a water filter pitcher (such as a Brita) to clean up the flavor a bit -- we haven't tested this but it's worth a try.
We used one 750mL bottle for each flavor of skittle -- five in all. Larger or smaller bottles will result in more or less flavorful vodka for the amount of Skittles given above.