Hi Mark,
This sounds like a great project!
Do you have a mold to get started? If not, you will need to create one. I sounds like you are making a rectangle, so a mold like this should be fairly straight forward. https://resinobsession.com/molds-mold-making/basic-mold-making-tips
You will also need to make sure your flowers are properly dried and sealed. We have a few articles on how to do that here: https://resinobsession.com/tag/drying-flowers
As for the resin, can you tell me what you would like to do with it when you are finished? i.e. do you plan to cut it, work it with power tools, etc.