Answer:
Sponge cakes are generally dry and lacking in flavor. That said, after torting your layers you will need to moisten the cake with a flavored simple syrup, preferably using a flavor that compliments the filling. For example, coffee/espresso syrup for a chocolate genoise with a mocha or chocolate filling. If you're using a berry filling, flavor the simple syrup with liqueur or extract that compliments the berry. Personally, I will often times use a syrup lightly flavored with lemon peel.
After moistening your layers you have two options: Let it soak for a day and add more (optimal flavor) or you may continue to decorate with buttercream or a frosting of your choice. For a smooth exterior, frost the sides with an even amount of buttercream then the top and, using even pressure and a sweeping motion, remove the lip of frosting around the edges. If the frosting isn't smooth, simply place the frosted cake in the ice box for about 20 minutes then, using a spatula dipped in hot water + wiped dry, lightly run the spatula around and over the cake to smooth the frosting.