If your machine is a clam shell platen; packing usually consists of one piece of card (about business card thickness) taped to the platen surface, 2/3 sheets of newsprint, or something of a similar thickness, another sheet of card then an oiled- manila top sheet. The card affixed to the platen is designed to ensure that when you spike two holes through from the top sheet, ( to position your make ready) there is no movement.These packing densities are based on a Heidelberg automatic platen and may differ for your machine but the basic principle should be similar. As Cleanwash mentions starting light with the filler sheets and building up to the required thickness is a good way to go. As you adjust your packing you will notice by looking at the back of the paper (holding it up to the light gives a clearer picture) that the impression may be uneven. If heavier at the top the platen is usually over packed if heavier at the bottom it’s under packed. The trick then is to balance the packing in tandem with the impression lever (if your machine has one) to get a perfectly level overall impression. Once even you can then patch up any lower areas with a make ready .Hope this helps. If you need any assistance with make ready please don’t hesitate to ask.