Combine the salt, salt substitute, sugar and cream of tartar in a large bowl or measuring jug. Add about half a cup of slightly warm water and whisk until all dry ingredients are completely dissolved and the liquid is clear with no visible crystals. The cream of tartar can be slightly stubborn. Keep whisking until the liquid is completely clear.
Add the fresh lemon juice and stir to combine. Top up with the remaining cold water to reach approximately 4 cups or 1 litre total. Whisk once more to combine everything fully.
Pour into your bottle using a funnel. The finished liquid should be clear and very pale yellow from the lemon juice.
Serve cold. The drink is best consumed after physical exertion in heat, which is the exact context it was designed for and the context in which it makes the most sense.