You must earn a total score of 10000 points during the duration of one magnet power-up. This is obtained by collecting corn when you have a huge multiplier, as the magnets only last 10 seconds or so. Using the technique I mentioned in "Chickens Can't Fly" to get gold medals on each level, this should come naturally.
Once you deploy a magnet, focus only on avoiding obstacles and collecting power-downs (to up your multiplier). Alternatively, if you earn a large multiplier (200+), you could then proceed to use a magnet and hope for the best.
This is probably easiest in the last two laboratories, Military or Physics as they contain ample power-downs. I would not attempt this in Survival Mode, as there is not that many power-downs, though it could potentially be possible.