Nice guide Pandaman.
I do it a bit different with the top two rows.
I would put the 1 and 2 in the correct space and then put 4 where the 3 should go and a random number (placeholder) where #4 would go.
positioning 3 under the 4, with the blank space under the placeholder.
Sliding the placeholder down, then 4 to the right, and 3 up so that the numbers are 1 2 3 4