When preparing whipped cream for frosting a cake, it's important to stabilize it with gelatin to help it hold its shape. Stabilizing the whipped cream is even more crucial when you're decorating with whipped cream icing; if you were to pipe regular whipped cream onto a cake, any shape it had wouldn't hold for very long, leaving you with shapeless blobs of cream on your cake. For the best results, use a piping bag with metal tips rather than plastic.
Chill a large metal bowl, the electric mixer beaters and the icing bag with metal tip attached in the freezer. Whipped cream comes together best when everything is very cold.
Dissolve 1/2 tsp. gelatin in 1 tbsp. water.
Make the whipped cream icing. To a chilled metal bowl, add 1 cup chilled whipping cream and 1 tbsp. sugar. Whip with the electric mixer, using the chilled beaters, until it starts to stiffen. Add the dissolved gelatin, and whip until the cream forms a "peak" when you pull the beaters away from the bowl.
Spoon the whipped cream icing into the chilled icing bag immediately after it's been made. Pipe a few practice lines on a piece of wax paper to get the icing flowing.
Pipe the whipped cream icing around the edge of the frosted cake, applying slightly more pressure to the bag in 1-second intervals. As you release the pressure, lift the bag slightly, then lower it again, to make a fancy border.
Place the tip about 1/4 inch above the surface of the cake to make a flower. Apply pressure to the bag, then release. As you release the bag, lower the tip slightly, then pull it straight up. Repeat anywhere you want flowers.