Hi Cindy,
Dried flowers look great in resin! The first thing you will want to do is seal the flowers to prevent water stains. I recommend our resin gloss sealer spray.
https://shop.resinobsession.com/collections/tools-and-supplies/products/castin-craft-gloss-resin-sealer-spray
Apply two coats, allowing them to dry thoroughly in between. The challenge is going to be getting the flowers in your mold. You really need to pour the resin first, then add the flowers. If you put in the flowers, then add the resin, you may trap the bubbles. Is your mold made of clear silicone? That will make it a bit easier to see through to place your flowers. If you don’t have that option, Im afraid the only way you can be sure to get your casting bubble free is to place the flowers in first, carefully add the resin, then pressure cast it.
We have a few articles on how to dry flowers and use them in resin here:
https://www.resinobsession.com/tag/drying-flowers/
As for the trays, you want to make sure you are using a resin that can be poured in large thin layers. The Alumilite Amazing clear cast is what I would use. You can find it in several sizes here:
https://shop.resinobsession.com/collections/resin/alumilite