Carefully peel off the zest of the 2 lemons and break up and crush the ginger. Add the sugar and cream of tartar and pour on the boiling water. Stir well. Remove and discard the white pith of the lemons, then cut lemons into thin wafers and float these on the water. Cover with a cloth and when cool sprinkle on the yeast. Re-cover and leave for 2 days. Strain off the yeast, strain out the ginger and lemon, bottle in screw-top bottles, leave for 3 days.