To create juice, activate the fruit press, right-click a fruit in your inventory, and choose "Create > Miscellaneous > Fruit juice". The resulting juice will be created inside the fruit press. Failure with fruit will decrease its weight. Certain juices at lower qualities (less than 10) can't be consumed. Needs more testing for exact value and juice types.
In general fruit juices will quench approximately 1% water for every 0.1 of weight.