Normally, to create the illusion of transparency against a single background is feat enough. In this image I “upped the ante,” by adding white roses above an existing floral arrangement that was already translucent. The second layer of roses makes an additional level of transparency: you can see through the white roses, as if they were a veil, and to the flowers below, which themselves only partially conceal the white backdrop.
One of the tricks with an image like this is to end up with something that looks painterly and elegant, not smudgy. If you try it for yourself, you may find that this is tougher that you might think!