Mix a cleaning solution of 1/4 cup water and 3/4 cup rubbing alcohol in a small bowl. Add a squirt of mild dish detergent for foaming action.
Dip a soft-bristled toothbrush in the cleaning solution and apply it directly to the pencil mark. Brush the area gently with the toothbrush to prevent loosening the quilt top fibers.
Wet a white cloth with clear water and blot the pencil mark and cleaning solution off the quilt top. If the quilt is new, wash it according to the instructions on the tag. Always consult a professional from the American Institute for Conservation of Historic and Artistic Works before washing a quilt if you are unsure of its value.
Hang the quilt to dry and prevent mildew. Use a fan set to a low speed setting to quicken the drying time.