As plants grow and the bulb begins to appear to be swelling, hill the soil around the base of the plant to exclude light.
Once bulbs are an edible size, generally at least the size of a flat tennis ball, carefully dig out of the soil or cut off at the base. If harvesting from a row, take care to limit disturbance to roots if there are other plants still growing. Leaves can also be cut to use as an herb.
How to grow fennel in a pot
Not recommended because a 300 mm pot would only accommodate one plant.