You can start this quest by talking to the Old Beldame in the Swamp Witch’s Hut area - be sure to ring the bell on her gate or succeed at a [Dexterity 15] check, as failing said check or barging onto her property will result in a fight.
Mind your manners and avoid a conflict, and once you’re talking to the Old Beldame pick the dialog option “I’ve got a question for you….”, and after her sharp retort respond with “How can I help you, good woman?” to start the quest Mushrooms from the Mud Bowl, which charges you with bringing the Old Beldame ten Black Rattlecaps, which can be found in the Mud Bowl area.
To get there from the Swamp Witch’s Hut area you’ll need to head east to the Kamelands, then north across the Shrike River - the area should have been marked on your map when you picked up the quest from the Old Beldame.
In the Mud Bowl, your only opposition is a Tendriculos, a shambler-like beast that prowls around the center of the area. Not only does the Tendriculos hit for between 20-30 damage, but it gets multiple attacks per round and has a high enough Attack bonus to challenge all but the most optimized tanks. You know how this plays out: if you attempt to fight the Tendriculos without buffing, you’re probably going to have a bad time. If you buff yourself (including the all-powerful Haste) you might just come out of the fight completely unscathed.
If you smite the Tendriculos you’ll be free to search the area for the ten Black Rattlecaps you need. No need for instruction here, there’s fourteen of them spread about and you only need ten, so you don’t even need to be particularly vigilant. Grab the mushrooms you need, then search some foul water along the western end of the map to find some loot, consisting of two Potions of Enlarge Person, a Potion of Blur, 89 GP and a Frost Battleaxe +1 among other, less valuable items.
With the ten Black Rattlecap mushrooms in hand, it’s time to leave the Mud Bowl and return to the Old Beldame to complete this quest: