Use fresh berries such as Blackberries, Raspberries etc, Basically any berry that stains your fingers will leave a mark on paper.
There are three components to making ink, these are Pigment (colour), Glue (vinegar, gum arabic, etc)) and a preservative (salt) but only for some type of inks.
You will need 1/2 cup Berries, the most common being Blackberries, Raspberries and Blueberries, 1/2 tsp salt, 1/2 tsp White vinegar, Water, Strainer, Bowl, Small ball of tin foil and some Small bottles.
Method 1. Place berries of choice in the strainer
2. Mash the berries using the ball of tin foil over the bowl to catch the juice
3. Add the 1/2 tsp white vinegar
4. Add the 1/2 tsp salt
5. Stir till mixed evenly.
6. Check the thickness of the ink by testing it on paper using your quill pen,
7. Add some drops of water if it is to thick, or add more berry juice if it is to thin.
8. When satisfied with the inks thickness, pour the finished ink into the small bottles..
9. Seal bottle with a tight sealing lid.
The finished ink can be stored in the fridge to prevent mold growth, and y the addition of white vinegar you will help preserve the ink.
Note: Some berry inks fade when they dry, if a darker shade is required just write over it again.
Since this ink is specifically made for use with quill pens, I think it might work with a dip pen, but I don’t
think it will be a suitable ink for use in fountain pens (use at your own risk).
Here is a video I found that will be helpful How to make ink from berries