You're going to need an oak tree. And some rusty nails.
I guess I should tell you what we're making. Ink. Iron gall ink to be precise.
It's the same ink used to write the Texas Declaration of Independence and the Constitution of the Republic of Texas. In fact, years it was the primary writing ink of the western world for 1500 years. It was only superseded my newer formulations around 1900 because old, reliable iron gall ink fouled newfangled fountain pens.
Back to the recipe. Go to your oak tree and find some oak galls, AKA oak apples. They're those globular growths you see on oak twigs. The result of a wasp laying it's egg on the tree and the tree's immune system encapsulating the invader.
Find a good handful and crush them into a powder. If you don't have a mortar and pestle, a hammer will do. The galls are used because they contain a higher concentration of gallotannic acid that the regular oak bark. (This is also one of the tannins used to tan leather).
Put your pulverized oak galls in a jar and add your rusty nails. Four or five will do. The rustier, the better. Add enough water to cover everything and leave the jar in a sunny place for ten days. Give the jar a good shake whenever you pass by.
There are alternate recipes, some involving wine or vinegar. In fact, there were about as many recipes as there were households, but this is among the simplest and produces decent results for the least effort. If it's too thin to use you can boil it down to the right consistency.
Now go find a goose feather and cut the end into a nib with your pen knife (that's why it's called a pen knife). Dip your quill in your ink and begin to write.
Your ink will be light brown or sepia when first applied, but over the course of hours or days, it will turn black.
Wait a minute, you say. Why is the ink on my old family letters dark brown?
That's because of the iron in the iron gall ink. Over time, if exposed to air, the ink literally rusts. This ink rust can cut right through the page, leaving holes where the ink was most heavily applied.
Still, you can't beat it's durability. Chemical analysis of the Dead Sea Scrolls has shown the ink used was iron gall.