Punch holes in the underside of the plastic jug for drainage, using a pair of scissors.
Cut the milk jug in half, to open up the top.
Fill the milk jug with 1 to 2 inches of gravel, to aid in drainage.
Fill the remainder of the jug with potting soil.
Plant your seedling in the milk jug, at the same depth as it was planted in the original, garden center container. Wet the soil so that it is moist.