The technique is simple: get a white diffuser, place it behind the flower on the sunward side so that it shades the flower. Set your camera to overexpose by at least one EV, make sure that no sunlight hits the lens and fire away. You may still need to adjust exposure in your photo editing software and maybe the white balance to make sure the colors are true.
Watch your depth of field. This is just a photographer’s way of saying how much of the stuff in the viewfinder will appear clear, independent of focus. I like using a shallow depth of field so that much of the subject is fuzzy and suggested. Another reason to limit your depth of field is to ensure that the diffuser remains out of focus. Just set your camera to aperture priority and select the smallest number, i.e. 2.8. Larger numbers, progressively create more depth of field. If you’re using a point and shoot camera, your lens may only go up to 8. That’s a lot of depth of field so keep the number small.
You can buy a diffuser at a photo store for a reasonable price. They fold, pretzel-like, into round zippered cloth cases. The first time you try to re-fold it could be a bit confusing, so save the instructions. Also, don’t unfold it near anyone or anything that is easily startled, like a cat. Diffusers tend to open with a pop, something that makes cats dig in all sets of claws and head elsewhere quickly. You could probably also buy some translucent white fabric and stretch it over a frame if you just want to play with this technique.
All this might sound complicated, but as long as you’re not still using film, experimentation is free. If you are using film, odds are that you know this already.
I’ve seen this technique used to make postcards, and it creates nifty screen savers and wallpaper, too. The icons show up well on the white background.