Start by finding a picture of a flower. The picture doesn't have to be a masterpiece. I just took a quick photo of a flower that I found laying around.
To start, create a new 8.5 inches by 11 inches document. Then go to File > Place, and find your flower pic on your computer. Roughly size the photo to your document the way you would like. Then choose Template from the pop-up menu of the Layers Panel. If the image becomes too transparent, double click on the layer to bring up the Layer Options Dialog. Here, you can change the Dim Images setting to whatever percentage you like.
