We need the Length, Width, Thickness, and Hinge(top of cushion to break or fold) measurement. Also where you want ties(regular or velcro) to be positioned, if any.

Styling or Construction Options:
-Knife edge- seam is on the middle of side
-Boxing - Separate piece of fabric sewn all the way around cushion edge
-Double Welt - Boxing with decorative rope on the 2 seams

Making Seat Pattern or Template :
-Position tissue paper on top of chair and trace around all edges with felt tip pen. Take care to trace inside any chair arms, back supports or upright slats in the chair back. To assure yourself a symmetrical seat cushion, fold the tissue in half lenghthwise aligning the tracings. Now cut out the pattern and see how it fits.