To make this simple project even easier, I've provided a template of the three pieces you need to cut from corrugated cardboard, along with directions for aligning the grooves/corrugations of the cardboard with each shape. Download that template here:
While that glue dries, go outside and take a walk. Collect dry pieces of ornamental grass, twigs, wildflowers, fern stalks, weeds — as long as the stem-ish part of whatever it is can fit inside of a corrugation tube in your cardboard, you're good.
xo and happy spring.