Make your pattern. Lay tissue paper on the seat of the chair and trace the outline with your pencil. Repeat for the seat back front, seat back rear and each side of the seat arms. On the outside edge of the seat arms, trace all the way down to where the arm meets the chair. Mark each pattern piece as you go with the name of the chair part and which side, right or left.
Add seam allowances to your pattern pieces. Use the straightedge and measure 1 inch away from the outer edge of one of your pattern pieces. Make a line 1 inch away from each existing line. Repeat on all pattern pieces.
Preshrink your fabric. Wash it in warm water and dry on a high setting. Iron the fabric and lay it flat on a hard surface. Pin each pattern piece to the fabric. Cut out each pattern piece on the outermost line. Remove the pins from each piece.
Pin the seat back front and rear pieces together at the top edge with the right sides together. Sew the top edge only with a straight stitch and a 3/8-inch seam allowance. Press open the seam with your iron.
Pin the back edge of the chair seat piece to the bottom edge of the front seat back piece with the right sides together. Sew along this edge with a straight stitch and a 3/8-inch seam allowance. Press open the seam with your iron.
Pin the arm pieces together with the right sides together. You should have four pieces -- two for each arm. Sew along the top and sides with a 3/8 inch seam allowance.
Pin the inside edge of each seat arm to the side edges of the chair seat piece with the right sides together. Sew with a 3/8-inch seam allowance and a straight stitch.
Turn the entire slipcover inside out. Pin the front and rear seat back side edges together. Sew with a 3/8-inch seam allowance.
Turn up the entire bottom edge of the slipcover ¼ inch to make a hem. Press this edge into place with an iron and pin into place. Sew around the entire bottom edge with a 3/8-inch seam allowance.
Turn the slipcover right side out and iron. Ease the slipcover over your lawn chair.