Make the cake as directed on the box. (In a 9x13 pan) then let the cake completely cool.
Once the cake is cooled, take the back of a wooden spoon, or the large end of a chopstick, and poke holes all over the cake. Space the holes about 1 inch apart from each other.
In a small bowl, add the strawberry jello and 1 cup of boiling water. Stir for about 1 minute until it's dissolved. (Tip: You can just microwave 1 cup of water for 2 minutes and it should be boiling then you can add in the jello.)
Once dissolved, add ½ cup of cold water to the jello and stir. Pour over the cooled cake.
Place the cake in the refrigerator for at least 3 hours or overnight. You don't want to cut into the cake before the jello sets or it will be mush.
After the 3 hours, combine the pudding mix and the milk and whisk until slightly thickened (about 2 minutes). Then add in the cool whip and fold together.
Spread the cool whip / pudding mixture over the cake evenly and decorate with the sliced strawberries and blueberries.
Notes
This cake takes at least 3 hours to set in the refrigerator so it is best make ahead of time or the night before you're planning to enjoy it!