Spring sq'irkjuice is made from Spring sq'irks, found in the Sorceress's Garden minigame. Level 25 Thieving is required to enter the Spring garden. To make the juice, pick 4 spring sq'irks and use one with a pestle and mortar while having 4 spring sq'irks and a beer glass in your inventory in order to make the juice.
You can also give the spring sq'irk Juice to Osman for 1,350 Thieving experience. Players wishing to gain fast levels in Thieving need only give Osman 40 glasses of Spring sq'irk juice to go from level 25 to level 45 Thieving. Progressing to the next available sq'irk juice provides better experience with fewer required sq'irks per drink.