Gather the materials you will need for making your ink.
Dump the berries into a strainer and hold it over a bowl to catch all the juice.
Crush the berries against the bottom of the strainer with a wooden spoon so all the juice drips into the bowl.
Squash as much juice as possible out of the berries through the strainer and then discard the remains.
Add salt and vinegar to the berry juice and stir well. The vinegar helps to preserve the color and the salt helps to prevent mold from forming over the juice.
Transfer the mixture into a jar with a tight-fitting lid. A baby food jar or small jelly jar is ideal.
Begin your writing project keeping in mind that the ink will dry quickly.
Freeze unused berry ink for later use. The berry ink will begin to smell badly if left at room temperature.