Needs to be done on a level where the blocks rain from the top of the screen.
I did this on World 1 Level 12.
A new blocks appears when the current block goes below the perforated line at the top of the screen.
As the block drops, lift it back to the top of the screen.
Try to position the blocks above free areas so they have maximum falling time.
Can be a little tricky, just keep an eye on blocks getting close to the bottom and lift them right back to the top.
Have 7 blocks falling at once and achievement is yours.
World 1, Level 12.