Fill grow bags with a quality potting soil mix until the soil reaches 1 to 2 inches below the grow bag's rim. The amount of soil you use depends on the size of your grow bags, which are available in sizes ranging from 1 gallon to over 15 gallons. Use your hands or a garden spade to mound soil towards the center of the grow bag. Create a slight grade that will discourage water from creating puddles at the grow bag's center.
Moisten the grow bag's soil until water drains from its underside. Use your hands or a garden spade to dig planting holes in the grow bag's soil. Create holes that are roughly equivalent to the size of the vegetable seedling's root ball.
Gently tap the sides of a seedling's pot or cup to loosen its soil from the container. Grasp the seedling's stem near a set of leaves and gently lift the seedling from its container. Place the seedling into its hole in the grow bag, fill in gaps with surrounding soil and pat the soil to firm in place. Water seedlings following transplanting. Place grow bags anywhere, including on concrete, as long as the site receives a minimum of 6 hours sunlight per day.
Making your own grow bag
Lay a bag of topsoil flat on the ground. Use a utility knife to cut an "x" across the surface of the exposed side of the bag. Plunge the knife through the bag at any corner and draw the knife through the bag until you reach the opposite corner. Repeat on the corner adjacent from your starting point to create an "x."
Unfold the flaps created by your cuts through the bags surface. Flip the bag over so the cut side's face rests on soil. Use a utility knife to score three lines along the length of the intact side of the bag, which now faces upward. Gently spread the bag's plastic at the cuts to expose the topsoil inside of the bag.
Use hands or a garden spade to dig holes roughly equivalent to the size of the vegetable seedling's root ball. Remove seedlings from their containers, gently place them into the holes and pat surrounding soil to firm them in place.
Surround seedlings with a one-half-inch layer of mulch. If desired, conceal the bag of topsoil with a thicker layer of mulch.