This is a basic tutorial on how to make Buttercream Roses. Keep in mind you can use the same technique in Royal Icing too. Both icings need to have a strong consistency so the roses don't melt. 
With Royal Icing you can let the flowers dry and use them at a later time. If your buttercream or royal icing is melting, make a fresh batch but stiffer. It will be more difficult to pipe them with an icing that is too soft and melts. The roses won't hold their shape.
In my recipe link I have both the Royal Icing and Crusting Buttercream recipes that you can use to make these flowers. Roses can be difficult to learn, that's why I used a lot of repetition in this tutorial. 

Practice is key... Have fun!