Orchids do not need to be pruned in the same way that you might prune a tree or cut back a bush or large flowering plant to keep it attractive looking. You shouldn't cut the leaves on an orchid. When orchids bloom though, you need to remove the dead blooms and flower stalks. The best time to prune back an orchid flower stalk is, for many varieties, in either October or November.  If you have an orchid that blooms several times a year that rule of thumb may not apply but late October to late November is usually a safe time to prune.

As flowers on an orchid blossom and then die you should cut each one off at its base. Occasionally it will bloom again after a flower or several flowers are removed. Once the flowers on an orchid have all died it's definitely time to prune. You'll know it's time because the flower stalk will have dried up and turned yellow or brown. The stalk should be cut down to where only about an inch of stalk is left protruding from the stem.

Some growers prefer to cut the stalk down when it's still green. When the stalk is still green it's possible that it might produce a keiki, or baby orchid.  There are two schools of thought. One is to wait and see if a keiki is produced. When the plant blooms again the new orchid blossom will be smaller than the original. The plant's energy will be sapped by the effort to bloom again. If you decide to cut the green stalk, cut it right above a node, or a small bump on the stem, leaving about 1/4 inch of stem above the node.  If the flower stalk is cut down while it is still green, no keiki will be produced, but the plant will have a reserve of strength to carry it over to the next blooming season and hopefully, to help it produce an even more beautiful blooms than the previous blooms.

Whether you plan to cut the flower stalk while it is still green, or to hope for a keiki, you should always use a sterile knife, scissors or a new razor blade to do the cutting. You never want to risk passing diseases from one plant to the next by using tools that are not sterilized.

However, keep in mind that dendrobiums do not need this kind of pruning. They will bloom year after year on the same stalk so cutting off their flowering stalk will leave you will a blossomless orchid.