To start the quest, talk to Santa just south of the grand exchange. Santa will tell you that Jack Frost is up to something and he wants you to investigate.



Head to the dwarven mine on ice mountain and talk to Jack. He says he won't share his plans with you.



There are several imps walking around the area, talk to the one called Benny. He will tell you to talk to Sir Taffy Cashien.



Travel to Falador park and speak to Sir Taffy Cashien, he will tell you about Jack's plan.



Go back to where Jack is and talk to him about his plan.



Head back  a choice: You can choose to help Santa or Jack.



After this, the quest is done and you will be rewarded with a christmas cracker.

Deliveries

You can now choose between delivering regular or dark presents with the deliveries you get from Santa.
You will get a dark christmas cracker from jack's deliveres. This cracker is exactly the same as the regular one,
so I recommend you just stick to doing regular deliveries, as it takes less time to get normal presents than it does to get dark ones.

If you need a guide for deliveries it can be found here.

Merry Christmas.