A flower press like this one is the best way to flatten thick flowers and leaves (roses, daisies, hosta flowers, etc.) but a hefty book also works very well for thinner kinds (clover, coral bells, geraniums, etc.).
All you have to do is pick a bunch of summer flowers, arrange them into the pages of a flower press or book, continue on with your eventful life, check on the flattened flowers several days later, and then slap 'em in a frame! It's amazing how the process even preserves the silky, iridescent, and fuzzy parts of the flowers. Now head outside and start gathering some blooms so you can have delicate floral art all year round!