Bake cake according to package directions. Allow to cool.
Using a toothpick, poke holes over the entire cake, all the way to the bottom. (Some people use the handle of a wooden spoon, some people use a fork. I like the smaller holes of a toothpick, and with a fork, you usually can't get all the way to the bottom, but it's your preference)
Prepare jello according to package directions, dissolving it in hot water, then adding the cold. Pour slowly over the cooled cake, making sure to cover the entire cake. It will absorb as you're pouring.
Put cake in the fridge to allow Jello to firm up, about 4 hours.
Frost with Cool Whip. You can top the cake with fresh fruit, if desired.