Make your way to Feldip Hills, and talk to Rantz, who lives east of the Feldip Hills, near the water (look for the quests icon on the minimap), (If you use the Fairy ring to Feldip Hills, it is combination AKS). He will tell you Skrach likes Jubbly Chompy but says before he will help you he wants to travel to Karamja to make some type of potion. He will tell you to meet him south-east of his cave.
Follow the coastline south and then east until you come to an old tree at the water's edge, near willow trees and a cabbage. Talk to Rantz, who will kick the old tree over and ask you to make it into a boat. Use your hatchet on it twice to make a crude boat. (You have to trim the roots of the tree back and then hollow it out.) Tell him the boat is ready, and he will shoot a steel brutal arrow to a tree on Karamja and tell you to cook a chompy "by da stabber!"
Note: If you don't already have some be sure to kill some chompies before you go to Karamja. (Raw chompies from the Grand Exchange work too). Also, don't forget to take some logs as they are scarce on Karamja.
(Side note: Because of an update in November 2012, as of now, you cannot attack the chompies in the minigame. This has yet to be fixed.)
The place of the treeThe palm tree with Rantz's arrow stuck in itTravel to Karamja. (If you take the Fairy rings, use combination CKR. Fastest way to get there would be to use a Tai bwo wannai teleport scroll) Head to the coast just north-west of Tai Bwo Wannai village and south-west of the General Store, and search along the beach until you find a small palm tree with an ogre arrow in it (Marked with a transportation symbol on the mini map). Make a fire under the palm tree, use your iron spit on a chompy, and cook the Skewered chompy on the fire. If you succeed, Rantz's kids—Bugs and Fycie—will bring the boat and take you back to Rantz.
Talk to Rantz again. He will tell you to make a balloon toad to use as bait:
First, fill your Ogre bellows with Swamp bubbles from the swamp just west of Rantz, and then use the bellows on a Swamp toad nearby to get a Bloated toad in your inventory.
Next, mine one of the 3 Piles of Rock directly West of the Swamp (near the Fairy Ring). Like the limestone east of Varrock, you can mine one pile three times for a rock.
Finally, with the rock, toad, and a ball of wool in your inventory, use the bellows on the bloated toad again to make a balloon toad. (Make sure your Ogre bellows is still filled with Swamp bubbles, or nothing interesting will happen.)
Drop (not release) the balloon toad in an area south of Rantz's Cave or in the area east of the Gnome glider (If no bird spawns try placing the toad by the blue flower in the grounds for catching the chompy in the quest Hunting Chompy Bird), and wait for the Jubbly (level 16). If it doesn't appear, you'll get your wool back, but you need another rock and will need to inflate a toad two times again.
When the Jubbly appears, kill it with your ogre bow. Pluck it like a chompy, take the meat, and cook it on Rantz's spit outside his cave.
It is possible to burn the Jubbly meat.
Return to Lumbridge Castle, and give the cooked Jubbly to Skrach (be sure not to eat it or you will have to get another one) to complete this part of the quest.
Able to use the boat trip to travel between Karamja and Feldip Hills. (You must pay a fee of chompies or exotic herbs that can be found on Karamja, such as rogue's purse.)
The ability to hunt Jubblies. (Counts for 1 point when hunting Chompies, though provides more range experience when killed, and Big Bones as a drop when killed, and the Raw jubbly gives more experience when cooked than a Raw chompy).