How can I make homemade ink?
I don't have a full recipe, but if you're fine with black ink, try to boil some crushed or grounded organic material containing large amounts of gallic acid (or other hydrolyzable tannins) with rusty iron and some vinegar (acetic acid). 

Suitable organic materials that come to my mind are oak bark or green tea.

You should end up with an ink that darkens over time. Note that this ink will probably clog fountain pens but should be fine for calligraphic pens.