Start off by getting yourself some 4 oz. (you'll need about 24) or 8 oz. (You'll need about 12) wide-mouthed jars. You'll want the variety with the straight side so that the cakes will slide right out. Otherwise, if there's a mouth to the jar, you'll have to just settle for spooing it directly out of the jar into your mouth... which can be okay too. *wink* *wink*
Coat the insides of the jar with Baker's Joy. This stuff is great, if not sometimes hard to find. It's like flour and butter all in one convenient aerosol can. If you can't find it... go the old-fashioned way: butter and flour the jars by hand.
Prepare the cake mix according to package instructions. Fill those jars up to about half-way full. Don't go more than that, or it will ooze over the edges of the jar while baking. The confetti cake happens to be B's favorite, and fortunately it makes for some pretty looking jars!

Arrange the jars on a baking sheet... just in case there's any over-flow. Pop them in the oven at the box's recommended temperature. I found that the 4 oz. jars take about 20 minutes to bake, and the 8 oz. ones about 30 minutes. But, I have a convection oven so, even with conversions, my times are often different than most other people's. You'll just want to check on your cakes frequently. I always keep long wooden skewers handy for BBQ'ing, and those are perfect for checkind doneness in the taller jars. Just insert one of the skewers into the center of the jar. Once it comes out clean, your cakes are done.
Now, here's where you have to make some decisions. Are your cakes traveling 3 days in the mail to someone a few states away? Are you taking them down the street for a neighbor's birthday party? Or are they being boxed up to be shipped thousands of miles away with no definitive delivery date? If you picked the latter, I recommend not icing your cakes. Instead, let them cool outside of the oven for 2-3 minutes. After that 2-3 minutes, immediately seal the jars. The remaining heat will actually create a seal to the lid. It will keep those cakes good long enough to reach their overseas destination, but isn't intended for long-term preservation. Throw in a tub of icing, some sprinkles, and a plastic knife with the box, and your recipient can ice their own cakes.
However, if an 8,000 mile trip to Afghanistan isn't in your cakes' future (and for presentation purposes here)... you can do the following:
Allow the cakes to cool completely in the jars. Once cool, slice off the muffin tops to the cakes so you have a smooth, clean top. I jiggle the cakes out of the jars a little ways because you'll actually want the tops of the cakes to sit about 3/4" from the top of the jar...

Now, it's time to make those cakes pretty. I take a tub of the whipped icing (it's easier to work with), and spoon it into a large Ziplock bag. Squeeze the icing into a bottom corner of the bag, and cut the tip off. Of course, if you have the forethought to actually purchase one, a pastry bag works just as well. Squeeze the icing, working in a circular pattern until the tops of your cakes are completely covered, and the icing is about 1/8" - 1/4" from the tops of the jars...

Add some rainbowliscious sprinkles...

Seal up your jars, and deliver some birthday wishes in a jar to someone you love!