What you need:
Measure the width of the car, and the length of fabric needed to cover the seats. Mark out a rectangle using those dimensions, using pins or fabric chalk.
Sew three towels onto shower curtain A, following the marked guide lines.
Measure the distance between the chair necks, and then mark out where the buckles should go.
With towel side facing down, sew shower curtain B on top, on three sides and 3/4, leaving about 7 inches of space in the last sides and leaving 1-inch gaps for the buckles.
Measure 9 inches for each strap. thread the strap through the buckles, then sew them on.
Push the buckles through the gap, then sew the strap in using the zig zag setting on the sewing machine.
Flip the hammock inside out through the 7 inch gap.
Use a blind stitch to hand-sew the remaining few inches.
Buckle onto the necks of the car seats.