I would draw a large circle (larger than the steering wheel, of course). Figure out the radius of the steering wheel, add for the amount to go around the back side of the wheel, plus seam allowance. Use a compass to draw your circle. Nancy's Notions has a Yardstick Compass for drawing larger circles - link:
http://www.nancysnotions.com/product...ortby=ourPicks - that you may want to consider. Sew as you would a quilt. Add elastic or drawstring - your preference - around the outside edge.