In a clean glass jar add juniper berries and sliced lemons (or lemon salt or juice). If adding sloe add it now also. Fill the jar to the top with filtered or boiled water. Stir very lightly, and tightly close the lid.
Place the jar in your fermentation spot (look below). Leave for 10-15 days. Every couple of days open the jar and stir the contents. You can taste it at this point too, and see if it's to your liking.
During the fermentation process, some juniper berries will sink to the bottom, or even travel up and down in the jar. As the days go by the liquid will eventually take on a minty-yellow hue. (If you put in blackthorn berries/ sloe, this hue will be reddish.) Try the juice and if it's tangy and on the sour side, it is done.
Strain the juice into a glass bottle(s). (Keep the berries, and set them aside.) Cork or cap the bottles and transfer them to the fridge. Keep the juice in the fridge for up to 2 weeks.
When getting ready to consume the juice, taste to decide if the sourness level is to your liking. The drink is healthiest as is, but if you find it too strong, you can dilute with water. Alternatively use mineral water, but this will add a little more carbonation to the juice. If you prefer it sweeter, you can add a teaspoon of sugar, or honey.