This part of the questline unlocks once you're asked to retrieve some jam.
You then need to head over to the second area with monsters, there should be frog like enemies, and head down the ropes. Click on the basket to talk to it, and finish the quests it asks of you.
The first quest requires 50 nom sandwiches, you can buy those from the shop or they can drop from enemies.
The second quest requires two condiments that will drop rarely from the enemies in that area, just use the auto attack function and let the hero afk a while, you can do other things or play as your other hero whilst you wait.
The last quest finally requires killing the enemies spawned by the picnic basket. This should be fairly simple, the monsters needed to kill all have a fist icon above them. Don't leave the room before killing them or they'll despawn, you can restart the quest via the method the game tells you if you accidentally despawn them.
With that, you're done! Give the fake jam you'll be handed over to the paper guy, and keep the real jam all to yourself.