Place fresh plant material between two sheets of paper, and then place something heavy over the paper — a few big gardening books, perhaps. Choose relatively flat plant material, such as leaves and ferns, as well as flowers that aren’t too bulky — pansies, for example. Once they’re pressed flat, you can use them right away or store them flat between sheets of paper.
Create a collage by tacking the leaves and flowers to a sheet of paper with small dabs of glue. You can frame the collage as is or photocopy it to make multiple notecards or framed prints. I’ve found that photocopied pressed flowers look very real and they won’t dry out and fade like the real plant material does.