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Attach the sandpaper to a flat surface and then rub the lapping plate over the sandpaper in a figure eight motion until flat.

The Norton lapping plate will work just fine as long as you soak it and lap it with some 400 grit wet/dry paper before taking the stones to it. The lapping plates are a waterstone themselves and...

At bare minimum, all you need is a barber hone or 10k+ stone. This will keep an already shave ready razor shaving pretty much for a lifetime barring an accident with the blade.