Glue in the magnets
Then use the toothpick to put a little adhesive into the Lego and place the magnet in the cavity by attaching it to the spoon and pressing the magnet into the Lego brick (see pictures below).
Note from the supermagnete team:
Since the magnets are practically the same size as the cavities in the Lego, you could leave the adhesive out.
You can skip the next but one step then.