Combine 1 cup peeled & grated ginger, with 4 cups boiling water. Strain. Alternatively you can boil the ginger until it reaches desired strength.
You can also run fresh ginger through a juicer, if you have one, but the result is much more potent, so you won’t need as much in the recipe.
Ginger juice keeps for at least a week in the fridge – this makes more than you’ll need for the recipe.