Wash and dry the cucumbers, removing any label stickers. The cucumbers should be thick, with blunt and rounded ends.
Cut one of the cucumbers one-third of the way from one end. For instance, if a cucumber is 12 inches long, cut it about 4 inches from one end. This gives you three different sizes of cactus to "plant."
Snip about 30 toothpicks in half. Push the picks into your cucumbers in a random pattern with the pointed ends poking out of the fruits. The toothpicks form the spines on your cacti. Don't place any spines on the bottom third of each of your cucumber pieces.
Fill a terracotta pot two-thirds full of sand. Set your cucumbers on the sand and fill the pot the rest of the way, firming the sand around the ends of the cucumbers. They should stand straight up in the pot.
Cut three or four 2-inch circles of brightly colored tissue paper. Lay them on top of each other and poke a toothpick through their centers. Pin the tissue circles onto one of your cucumber cacti near the top. Pinch and ruffle the tissue to create a cactus flower.