Measure the dimensions of your chair. You must get the width and height for all the fabric-covered panels. Record these numbers on a pad, noting which part of the chair they came from.
Lay the sheet you want to make a slipcover from over the chair. Press the sheet firmly to the sides and into the seat crevice. Have a length of fabric draped down past the bottom of the chair about 4 to 5 inches.
Pin the sheet to the panels, placing a pin every 2 inches on the periphery until you have an outline of the panels on the sheet.
Mark the sheet with a pencil along the edges of all the panels. Take the pins out and remove the sheet from the chair.
Mark the inside of the sheets to match the part of the chair they came from. Cut along the lines and remove the panels from the sheet.
Sew the panels together where they should join, using your previously recorded measurements to ensure that the fabric is not bunching up or being stretched out of shape. Test for fit periodically by placing the in-progress slipcover over the chair and seeing if it fits. Cut out the threads and resew if the slipcover is not fitting properly.
Hem the bottom strip of the slipcover or tack it to the bottom of the chair depending upon what look you want for the slipcover.