Peel the lemons and place them in a jug or bowl with the sugar. Pour the boiling water over it, then squeeze the lemons in a separate bowl. Add the tartar and stir well. Now add the juice to the syrup and stir everything well again.
Let the liquid cool down and then pour into bottles.
The syrup can be infused with all kinds of water, if you prefer it fizzy, you can then mix the syrup with mineral water instead of still water.