I would use a product designed specifically for marking on fabrics. I would not use a pencil, too many reports of people not being able to remove it. I think any quilt marking pen or chalk would work, just make sure you read the product instructions very carefully and KEEP the packaging! Some products remove with iron, some with water, some are made permanent by iron, and you don't want to get them mixed up!
The Clover water-soluble marking pen works well, you mark, sew, then spray with a fine mist and the ink disappears. Sewline also makes one that will fade away after about a week, or you can make it disappear immediately with water.