This is a cheap & dirty but also a highly effective way of lapping engine parts.
Use a glass on a flat and rigid surface and set a silicon carbide sandpaper or fine grinding compound on top of that. The surface should be true and this is where the glass kicks in but a granite surface plate is a better choice.
Using slight downward pressure and 8-pattern movement, the parts accommodate a flat and smooth surface finish. A smear of WD40 does help the whole process when using sandpapers or fine emery clothes.