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When my phals are done blooming I usually wait till the stem turns brown. The energy in the stem is still used by the plant (at least that's when I've been told). I cut it just above the bottom node. This is a joint that is covered with a little flap. Depending on your phal you might get another spike from the old one if it is still green. Sometimes mine do spike again but not always. When you cut it put a little ground cinnamon on the cut. It will keep infection away. I also always use a new single edge razor or sterile clippers to make the cut. Hope this helps.
While a lot of people wait until they turn brown you can cut it off earlier if you want to. I sometimes do if I don't want the old spike looking messy on the plant, other times I leave it on until it dries.
If you cut it when green then cut it right down near the base, as close to the plant as you can without damaging the plant. Dabbing a little cinnamon on the cut is a good precaution although I don't always do it.