You can dry flowers in silica beads or in a very low oven or in buches hung upside down. I have tried all these methods with varying success. The method that worked best for me was pressing the petals and leaves in a press in between blotting paper. This method kept the best colours too.
I used PVA glue to stick them down.
It's best to put any artwork you do behind glass as they can take moisture from the air and start decomposing. I'm not sure if PVA or Gel Medium would work as well but it is worth a try