Paper
that has been soaked and blotted. This paper was cut, soaked overnight in distilled water, then laid out one by one with a coulple of sheets of paper towels between to separate them and soak up excess water.
Something flat is laid on top and weight is added to press the paper and towels so the water is squeezed out further. A screw press can be used for this.
This takes a few hours. I have left them overnight and they were fine.
When you put them on the plate they should be evenly damp.
Want to see this copper plate in the several stages of being engraved before printing?