To make a custom baby seat cover, pin and cut the fabric, attach the cording and lining, make the straps, and sew the straps to the cover. The required supplies include fleece, pins, cording, lining fabric, a pattern, shears, Velcro and a sewing machine.
Lay the fleece on a flat surface with the wrong side up. Place the pattern pieces on the fabric, pin them in place and cut the fabric. Remove the pattern pieces and pins, transfer them to the lining fabric, and cut out the lining. Remove the pattern and pins, and set the strap pieces aside. Lay the remaining pieces on your work surface.
Pin cording around each piece of fleece, and sew it in place. Lay the fleece right side up, and pin the lining over it with the right side down. Attach the lining with straight stitches. Keep the sewing machine foot pressed against the side of the cording as you sew. Leave a small gap so you can turn the cover inside out.
Remove the cover from the sewing machine, pull out the pins, and turn it inside out. Close the gap with hand stitches. Return the cover to the sewing machine with the top side up, and stitch around the cording. Cut the thread, and set the cover aside.
Pin cording around the fleece strap pieces, and sew it in place. Pin the strap lining to the fleece with the right sides facing in. Sew the lining in place, leaving a 3-inch gap at one end of each strap. Turn the straps inside out, and hand-stitch the gaps. Sew strips of Velcro across the ends of the straps. Stitch the rough (or "hook") side of the Velcro to the strap lining, and sew the soft side to the exterior. Sew the straps to the cover.