Place your paper in front of you, lengthwise. Draw a line through the center of your paper, bisecting it horizontally. Place your protractor against the right side of your line, and start a line curving about 60 degrees from the bottom of that bisecting line. Extend your curved line all the way to the left side of the paper. You just created a long, curved right triangle shape on the bottom half of your paper.
Fold the paper along the bisecting line. Fold the paper so you can see the curved line you drew. Cut along that curved line: You should be cutting both layers of paper, if you folded the paper properly. You now have a triangle that resembles a cone with outward sloping sides.
Turn the cone so its flat bottom faces you. Cut off the bottom 10 inches of the cone. You now have two template pieces: one 10 inch by 20 inch rectangle and a 20 inch tall cone.
Lay out a T-shirt across your work area. Place your rectangle template over the front center of the shirt. Outline the template with your chalk. Cut out your outline. Repeat the process with the back of the shirt, but use the cone template this time.
Place your two T-shirt pieces next to each other, imitating the way the template looked before you cut it into two pieces. Pin the two pieces together along their wrong sides, which are the sides that don't show the full color or design of your T-shirts. You want to hide the stitches on this side. Leave a 1 inch seam. Use a straight stitch, or a stretch stitch, if your machine has one.
Make another large cone fabric piece out of another T-shirt. Pin together the two large cone fabric pieces you've constructed along their straight edges, on the wrong side and leaving a 1 inch seam. You've completed one section of your bean bag's cover.
Create six curved sections. Pin these curved sections together along their arced edges, on their wrong sides; again, use a 1 inch seam allowance. Sew along these lines, using the same stitch you used before. Leave a 6 inch opening in which you can add your bean bag's stuffing.
Pack your beanbag chair with plastic bags, styrofoam peanuts or cut-up upholstery foam. You can raid old, worn out pillows for their stuffing, too. Hand-sew the open seam closed with matching thread.