Lay the ruler flat near the edge of the paper. This will mark the tear point for the deckle edge.
Draw a pencil line along the ruler. Apply minimal pressure, because you don't want to use an eraser on the delicate deckle edge.
Dip the paintbrush in water. Tap the brush to remove excess moisture. Paint the pencil line with water.
Wait a few minutes for the water to soak in, but not so long that the paper dries.
Hold the ruler down again, along the pencil line, and begin pulling away the paper. Start at the bottom and work up, applying steady pressure, but try not to pull too hard. Paper that is very thin and light may pull away without the help of the ruler.
Wait for the deckle edge to completely dry before using the paper. Repeat the process for each edge you want to deckle.