You will need a stick of solid ink and an inkstone, these are usually made of slate, it is possible to use an unglazed porcelain saucer instead of an inkstone.
Add as much warm water to the stone as you need to make the quantity of ink you want. Remember the more water you add the longer you will need to grind the ink stick to get the desired blackness. Warm water works more quickly than cold.
Grind the stick in the water until you achieve the colour of ink you desire.
You can see the depth of colour change as the stick is ground longer.
It is important to dry the stick when you have finished making up your ink to prevent it cracking and crumbling. If you have ground more ink than you need it will keep in a screwtop container.