You'll need a cucumber that is nice and thick, about two inches in diameter, and at least a couple of inches longer than your dick. At the height of the season, you shouldn't have trouble finding a large one. Needless to say, you should pass over the expensive (and too slender) cellophane-wrapped English cukes.
Cut both ends off the cucumber. At one end, be sure to expose the full thickness of the cucumber's flesh. Dig into it with a spoon and/or knife, and hollow out the seeds and interior. You'll need to cut away just enough of the inner wall that you can slide your cock in. You want to keep it tight, and you want the cucumber walls to hold up. It doesn't have to be perfectly smooth inside. Some ridging on the interior can add interesting texture.
You can tunnel straight through so both ends of the cuke are open. We suggest keeping the opening at the far end narrower, so you can block it off with your fingers or palm for more suction on the out-stroke.
Warm the cuke to body temperature by immersing it in hot water. You could pop it in the microwave, but for something this small, we recommend water submersion instead. Nuking it for even a few seconds could make it hot enough to burn you, or cook it and make it unusable.
Slide into your cucumber's naturally slippery juices, or lube it up with a squirt of vegetable oil, and enjoy the salad days of summer.