If I need to substitute 2tbsp of instant coffee for brewed black coffee, how much should I put in/ how much of the other liquids should I take out to compensate?
Why not just prepare it based on the instructions on the package (add water, mix), and then use it exactly like you would the normal coffee?
You mention in the comments that you actually need it the other way around (replace instant coffee with brewed). I was at the store recently so I took a look at instant coffee and all of them recommended between 1 and 2 teaspoons of instant coffee powder to 6 oz. of water (3/4 cup). So for 2 tbsp of instant coffee you'd need 4 1/2 to 9 cups of coffee to replace it (depending on how strong you want the coffee flavor to be).
If there's not that much water in the recipe already, it's probably why it uses the powdered version and I'm not sure there's anything you can do.
It is 2 tsp instant coffe for every 8 oz of water. To brew correctly you should use 1 tbs of ground coffee for every cup of water. All you need to do is brew the coffee directly into the liquid in the recipe( way easy with a French press, or put a funnel into a container, line funnel with coffee filter and pour hot liquid over) and use above numbers to figure out how much coffee you need to use.