Winter sq'irk juice is made from Winter sq'irks, found in the Sorceress's Garden activity.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 Sq'irk garden, you will also find an 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 10% of your energy (doubled from 5% on the 8 July 2009 update).
You can also give the juice to Osman for 350 Thieving experience.