I did it with Miss 4 and Mr 6, who both had an absolute ball collecting different things from the garden and taking turns whacking them until they were a semi pureed blob. F.U.N.
The general technique:
1. Take and old flat wooden board
2. Put some heavier paper on the board (the heavy paper withstands hammering better and absorbs more of the plant colour)
3. Grab some leaves and flowers from the garden (glossy leaves don't work very well)
4. Put the samples on the paper and cover them with 2 layers of kitchen towel.
5. Hammer everywhere on the sample but keep your fingers well out of the way of enthusiastic hammering children!
6. Remove the paper towel and the sample and admire your print.
We didn't have a fancy hammer, so we just used an ordinary, smallish one.
Just think of the fun you could have making pretty cards for Mother's Day or a grandmother's birthday.