On a cutting board, set the oranges on their side. Using a sharp knife, trim off the tops and bottoms of the citrus, being careful not to cut too much into the flesh. Set them cut side up. Using a sharp knife and working around the oranges, cut between the flesh and the pith, following the fruit’s contour. Discard the peel and pith. Using a paring knife and holding the oranges over the bowl with the cucumbers to catch the juices, slice between the membranes of the fruit and use the blade of your knife to remove the citrus segments.