Create 2 more layers. 1 for adding mildly black areas and another one for strongly burned lines to draw cross on the paper. Make it "Color Burn" style in "Blending Options".
Step 4:
We keep adding layers to make the paper older and older. Add one more layer for wet stains. No special style for that. Just playing with opacity of your black brush. 24% was fine for me.
Step 5:
Now it is time to add some volume to our sheet. It can be easily done with drawing some white areas.
