In our house, it can take a bit of imagination to make any activity seem more enticing than screen time. Telling the kids they can make pictures by bashing flowers with a hammer, usually works a treat! This craft activity is great fun, here we used flowers from our garden, but I've also seen fab results over on Pinterest where people have used leaves.
It really is as simple as positioning your chosen flower on the paper, covering it with kitchen towel (we use 3 sheets), and hammering away, making sure you work across the entire area that the petals are positioned under.
We have found that flowers with thin delicate petals work the best. The blue flowers I believe are a type of rock cress, and we have also found that Wall Flowers work well too. Part of the joy of this activity is trying out lots of different sorts of flowers, so I don't want to put in too many spoilers. I will say, however, that Chive flowers don't look nearly as attractive, once they have had a hammer taken to them!