Iron your canvas flat then lay it on a hard floor wrong side up and draw a trapezoid on it with the following dimensions (refer to diagram):
- front edge of the chair (this side is along the selvedge of the canvas) - 137 cm (54")
- back edge - 71 cm (28")
- each side - 135 cm (53")
- approx. perpendicular distance from front to back - 130 cm (51")
Draw on the canvas using chalk, a crayon or a biro, whatever you have.
In the photos you'll see that the back edge of the chair is along the selvedge, but it would have been better along the front to make for a smoother front edge.
Cut out the trapezoid. Don't throw away the scraps, you can use them to protect the tree from the rope and to make a head cushion for your chair.