How do i determine the resistors needed to reduce a 9 volt battery to 2.5 milliamps? Answered

With Division!

In all honest, if you can't understand Ohm's Law, maybe you should take a step back.

You can not reduce voltage to current ! !

You can limit the output current flow of a battery
by putting a resistor in series with the battery.   R=V/I=9/.0025=3600 ohms

BUT as the battery discharges your resister will also lower the cranial
milliamps you so desperately need to think you are evolving. .

You should use a superior constant current regulator like
an LM317 three pin IC.

This will deliver a 2.5ma cranial current down to a 5  volt dead battery.

A