To Prepare Fresh Lemongrass: Pull off any outer leaves that are dry or overly fibrous. Cut off the bulb and discard. Now slice up the lower 1/3 of the stalk. For this version of Tom Yum, the lemongrass will not be eaten (this is usually the case in authentic Thai cooking), so you needn't slice it thinly - thicker pieces will be fine. Use the upper stalk as well by cutting it into 4-inch lengths.
Add your prepared lemongrass to the stock (including upper stalk pieces, if using fresh lemongrass), and bring soup to a boil.