Print out a bean bag pattern, such as the one featured in the Resource section of this article. Check the scale on the pattern and draw a pattern to scale on a large piece of tracing paper. Use the smaller pattern for a child size bean bag chair and the larger pattern for an adult size bean bag chair.
Fold a large piece of paper in half and draw an arc around it to create the top piece of the chair. Repeat this for the bottom. The arc for the top piece should measure 5 1/2 inches for the adult chair and 4 1/2 inches for the child's chair. The bottom piece arc should measure 12 inches for the adult chair and 10 inches for the child's chair, according to the Singer Reference Library. Cut out the two arcs and unfold the paper. Press the circles flat.
Pin the patterns onto your fabric with straight pins and cut out the pieces with fabric scissors for neatly trimmed edges.
Measure in 1/2 inch from all sides and sew pieces together with the wrong sides of the fabric facing outward. The top circle goes on the smaller curve at the end of the bean bag pattern piece and the larger circle goes on the larger curve. Pull the sides of the bean bag in toward each other and sew. Sew a zipper along one side of the bean bag. Make sure you're still working on the wrong side of the fabric so the zipper will go inside the bean bag when you turn it right-side out. Sew the bottom and top pieces in place.
Unzip the zipper and turn the bean bag chair right-side out. Fill the chair with polystyrene pellets and zip it closed. Place the bean bag chair on the floor with the zipper facing downward.