You must be careful to prepare your flowers before using them on the cake to make them safe and not contaminate the cake.
I usually cut the flowers short and give them a soak in a tepid clear water bath to remove any dirt or grime. Wash your own hands throughly with soap and water before handling the flowers.
Some flower stems leak sap, thus you must be sure to seal off the cut end of all flowers before using them on the cake.
Make a clean cut with a sharp, cleaned knife.
You are going to cut your flowers at various (short) lengths because you want to stagger the flower stems at different heights on the cake.
I insert a clean toothpick in the cut end of the flower - in this case a standard rose.
Open a new roll of white tape that hasn't been used before.
Tape down the flower stem, covering the bare green stem so it blends in better with the white icing.
Tape down tightly to the top of where the toothpick is inserted in the bottom of the flower stem. Make sure the cut of of the flower is sealed with the tape.
Leave the bare wooden toothpick exposed. This is the part that will actually be inserted into the icing. Lay your picked flower in a clean cake pan that you will store in the fridge until ready to decorate the cake.