Winter sq'irkjuice is made from Winter sq'irks, found in the Sorceress's Garden minigame. There is not a Thieving level required to access the Winter garden. To make the juice, pick 5 Winter sq'irks and use them with a pestle and mortar. You must have 5 Winter sq'irks and a beer glass in your inventory in order to make the juice. In the Winter garden, you will also find a herb bush that allows you to pick two herbs at a time.
Picking the Winter Sq'irk gives 30 Farming experience. If you make the juice you get 12 Cooking experience. Drinking the Winter Sq'irk juice will restore 5% of your energy.
You can also give the juice to Osman for 350 Thieving experience.