Washi tape that mimics the shape of a stamp! NOT a real stamp.
I recommend using scissors to cut each stamp if you want to use them individually, to get the cleanest line possible. The washi tape is easy to rip by hand, but this method may not yield the cleanest "stamp" lines.
Please be careful when starting the roll of washi, as it's delicate! Once you get it going, it'll roll out easily.