I think what you mean is, how do you make the flowers raise up so they are not just sitting flat on the cake? If that's what you're talking about then what you need to do is make a mound of icing (I just use the open coupler on a bag of icing either the color of the top of the cake or sometimes you can use the same green you use for the leaves) using pressure to create a larger globs of icing, just squeeze out a mound somewhat smaller than your bouquet of flowers will be, making it around 1 to 1 1/2 inches tall in the middle, tapering off to just a little higher than the flat top of the cake on the edges.Then just place your flowers directly on this mound starting at the center. Once they are placed add your leaves. This adds dimension and really helps give the flowers a more natural rounded appearance.
You really don't need toothpicks or spikes piped on the back for the top of a cake. If you use wires you don't need the mound of icing because the wires are holding the flowers up in the right position.