You first make up the concentrated solution with MagHydroxide in cold fizzy water.
You put it back in the fridge and shake it a bit 30mins later maybe again if the sides of the bottle haven't gone crinkly or the mix is still cloudy.
You then add a third of the concentrate into the 4litres of plain water.
So you now have 4anda third liters of magnesium rich water.
I then keep this in the pantry and use it to fill a sodastream bottle that I fizz and keep in the fridge. So I have sparking magnesium rich mineral water for next to nothing.
You don't have to refizz if you prefer plain unsparkling water.

Further reading

Indications for a medium mineral high bicarbonate water (Cerelia) in urology.

Dissolution of radiolucent renal stones by oral alkalinization with potassium citrate/potassium bicarbonate.

Bicarbonate, but not potassium, had a favorable effect on bone resorption and calcium excretion. This suggests that increasing the alkali content of the diet may attenuate bone loss in healthy older adults.