Choose a T-shirt that fits well. Lay it flat on a large piece of paper, cardboard or wax paper. Pin the corners of the shirt into place to help the fabric lay flat. Trace around the shirt, leaving 1/2 inch for a seam allowance.
Slip a piece of carbon paper under one arm seam with the colored side facing down. Run a tracing wheel along the arm seam, pressing hard enough to make a mark onto your pattern paper. Repeat with the other arm.
Remove your shirt. Cut along the bodice lines, and cut out the sleeves. Pin the bodice piece to a second sheet of paper, and trace around it. Add 1/2 inch at the sleeve openings for a seam allowance. Pin one sleeve piece to another sheet of paper, and trace around everything but the straight edge that was the outer edge of the sleeve. Flip the paper along that line, and pin it. Then, trace around the edges to create the full sleeve pattern. Cut out the final pattern pieces.
Cut two each of your bodice and sleeve patterns from a piece of knit fabric. Sew the bodice together at the side and shoulders. Sew the sleeve pieces into the arm holes, bringing the two narrow ends of each piece together at the underarm. Hem the shirt. Finish the neckline by rolling the edge under by 1/2 inch and sewing it into place. Repeat to reinforce the seam.