You have to grind the ink stick until the ink is thick and deep black. To know when the ink is thick enough take one drop of ink with the stick and put it on the rim of a white saucer. When the ink does not run down the saucer, it is ready for your session of Japanese calligraphy.
If the ink is too thick, the writing will not be fluent, because the ink does not flow smoothly from the brush. On the other hand if the ink is too thin, the ink will flow down too fast.
Only with practice will you learn to determine the perfect thickness of the ink that you want. The type of paper you use, the style of Japanese calligraphy, your brush, the pressure you make on the paper, and even the weather are factors that should be considered when judging the thickness of your ink.
The process is pretty much the same with Sumi painting. You simply grind a water paint stick against your stone instead of ink. (Fineartstore.com has a nice assortments of primary calligraphy and Sumi supplies if you cannot get them in your hometown, or there are some very dedicated shops like Silverdragonstudio.com, artmam.net, Paragon-art.com, and Orientalartsupply.com where you can find some really great items to assist you in developing your own way with the Four Treasures.