A medium bag uses (4) 18x18-inch pieces and the finished bag is two layers thick.
Pin and Sew Two Sides
Pin and sew seam on Side A, stitching all 4 layers together.
Pin and sew seam on Side B, stitching all 4 layers together.
Rotate and Pin
Rotate one-quarter turn placing Seam A in center and Seam B directly underneath.Pin in place.
Pin and sew seam across bottom.
Rotate Seams Back and Prepare Bottom
Shift everything back so Seam A is on left side and Seam B is on right side.
Align bottom points C1 and C2 with center of top edge.
Draw and sew seams for Bottom E and Bottom F at middle points of edges.
Turn Bag Right Side Out
Turn bag right side out and push out corners from inside.
Finish Top Lip
Pin and sew seam around entire top of bag.Fold lip over (like a sock) an inch or two and stitch in place.
Add Handles (Optional)
Cut and hem two 18-inch lengths of webbing.Attach ends of webbing on both sides of Seam A and Seam B, positioning them equal distance on either side of each seam and from top edge of bag.
Notes
See the diagrams in the article for more details. Article also has suggested sizes for mini, small, large, and extra large grow bags in addition to the medium size.