When preparing whipped cream for frosting a cake, it's important to stabilise it with gelatin to help it hold its shape. Stabilising 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 waxed 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.
If you want to add words on top of whipped cream frosting, a buttercream or icing sugar icing works best.
Do not overwhip the cream; it will separate, and you'll have to start over.