There are 30 pressed flowers to collect. They appear on your map/minimap as a flower icon. You can buy a map from a shop to help you with finding them. I am including the names of each and which borough they are found to help you if you need to find ones you are missing. To check which ones you have found, go to Main Menu > Database > Encyclopedia > Collectibles > Pressed Flowers.
Whitechapel (3) - Amaryllis, Laurustinus, Poppy
Lambeth (4) - St John's Wort, Melancholy, Sunflower, Rose
Southwark (4) - Violet, Mimosa, Broom, Thrift
Westminster (6) - Mignonette, Daffodil, Strawberry, Persian Iris, Yellow Day-Lily, Anemone
The Strand (5) - American Cowslip, Dwarf Geranium, Love-in-a-Mist/Devil-in-a-Bush, China Pink, Herbaceous Heath
City of London (5) - Snowdrop, Forget-Me-Not, Common Jasmine, Jonquil, Lily
The Thames (3) - Wallflower, Gum Cistus, Yellow Lupine
video courtesy of COG Inc on YouTube