Buy silk roses (available at crafts shops) and remove the petals from the stems, trimming them with floral shears if necessary -- or buy loose silk petals.
Using the foam paintbrush, dab a thin layer of glue around the top rim of the container. Press the base of the first rose petal onto the glue, letting the top of the petal extend over the rim; press additional petals onto the glue, overlapping one over another.
Once you've covered the rim, brush a small amount of glue over the bases of the petals, for a secure hold (the glue dries clear). Brush another thin layer of glue on the outside of the container below the attached petals, and press on more petals by their bases, concealing the glued portion of the top layer with the tips of the petals. Don't press any petal tops into glue. Continue until the container is covered. At the bottom row, tuck petals underneath, dabbing a bit of glue on the container's side and base.
Fill a sink with cool water. Using flower pruners, cut stems underwater at a 45-degree angle, so they won't sit flat on the bottom of the container, for maximum water absorption. Immediately place stems in 2 inches of warm water (no hotter than 110 degrees). After 5 minutes, transfer them to the vase filled with cool water. You should keep the arrangement in a cool area to help the roses last; if you want them to open quickly, place them in full sunlight.
Plastic yogurt cup, or another container about 6 inches tall, such as a paper cup or a terra-cotta flower pot
Disposable foam paintbrush
Silk rose petals
Spray roses (and other flowers, if desired; we added sweet peas to one arrangement)