One serving = 1/4 cup of the ginger beer concentrate + 1 1/2 cup sparkling water. Wash and cut the ginger root into medium chunks. Place it in a blender together with 2 1/2 cups of water.
Mellissa of I Breathe I'm Hungry made a sugar-free ginger syrup that can also be used for making ginger beer. In my recipe I used Sukrin Gold which is my favourite brown sugar substitute, and used a blender that allowed me to extract a lot more flavour from the ginger root.
This recipe makes a semi-sweet alcoholic ginger beer of about 4.5% strength. You can vary the sweetness by varying the amount of lactose in the recipe or make it dry by leaving the lactose out altogether (see notes)
You may have found some ginger beer kits, but they can have an artificial taste and are lacking in spicy ginger bite.