Using flowers in cake decor is a very effective, modern technique. First of all you need to make sure the flowers you are using are not poisonous. You can search on the internet or ask a florist.
It's strictly prohibited to use this flowers in cake decor:
  • Jasmine
  • Lily
  • Tulip
  • Lily-of-the-valley
  • Hydrangea
  • Strelitzia
It's recommended using this flowers:
  • Gypsophila
  • Rose
  • Orchid
  • Daisy
  • Tiger lily
  • Cornflower
  • Eustoma
It's better to buy the flowers a day before using them to decorate the cake. Cut them all to a certain length (5-10 cm), and leave them for at least 4-5 hours (or overnight) in cold water.

The next day, you have to wash the flowers and clean them from all the unwanted petals and leaves. After you've done that, take 60% alcohol (or higher) and mix with water (7 parts alcohol, 3 parts water). Spray it on the flowers, to kill the bacterias.

The next step is creating the flower composition we want. After you've gathered a bouquet or picked the decor elements you need, wrap the stems in ribbon (you can buy it from the flower store). When stretched, it glues and keeps the stems together very well. An additional plus to this ribbon is the fact that it protects the flowers from drying out, this way the flowers can better survive the day.
The last necessary step is wrapping the stems, from bottom to the bud, in plastic wrap.
Your flowers are ready! Until the cake is done and prepared, you can keep the flowers in the fridge.
Examples of cake decors with flowers (photos are taken from Pinterest) :