...just some ordinary erasers, ink/paint and paper
i cut the tree out of an ordinary white eraser with my exacto knife, used a pencil eraser for the dots and stamped onto ordinary brown packaging paper
tip: make sure you have suitable ink/paint and paper - test it out first...so that the ink/paint soaks into the paper and doesn't smear. usually really smooth or glossy paper doesn't work well...
p.s. ikea has this really cheap brown paper, but i haven't made it to the store yet, so i can't tell you how well it works.