Put the paper in first, then soak it before you backfill, otherwise it will draw the moisture out of the soil. Personall I would prefer to add the shreddedpaper to either the compost heap or (better) the leaf pile, and use the composted material for planting the potatoes. Incidentally, I have always believed that there is little point in placing compost etc under the seed tubers. The roots grow from the collarettes around the base of the new shoots and mainly fan outwards, so I always place my compost or leaf mould around, and even over, the seed tubers. This has the added advantage that when I dig the potatoes they come up much cleaner.