Get a nice heavy duty mortar and pestle, and a digital scale for weighing meds. Crush and weigh the pill. Remove the percentage you want to taper.
For example, if your pill weighs 100 mg and you want to reduce by 5%, remove 5 mg, take the remaining 95. The next time you reduce the dose, take 5% away from 95 - 4.75 round up to 5), remove 5 mg again and take 90. And so on.
You can also do a liquid taper, I'm not as familiar but same concept, you can reduce by very small amounts. See Ashton Manual