Side Note: I would have loved to have done this for my wife's wedding bouquet - and even more recently for the flowers my daughters carried as flower girls in a dear friend's wedding. Very cool concept!

People are far more likely to believe what they can see. Produce a short video that shows your process - from start to finish - so they can see the magic happen. It can be quick clips of each stage showing the exciting moments (like putting the flowers in, and sealing the glass). This doesn't need to be (and probably shouldn't be) more than 30 seconds.

Example Storyboard:

Scene 1 - We take amazing flowers (3-5 seconds)
Scene 2 - and preserve them by "xyz" (10-15 seconds)
Scene 3 - and seal them in glass to keep them beautiful forever. (10 to 15 seconds).

Hope that helps!

Cheers,

Ryan