I go through periods of high and low caffeine intake, so I have some experience in managing withdrawals.

Let's say you want to go down from 4 cups to 2 cups. Here's how to do it:

1. Write down how much you currently use. Let's say you're using the traditional 60 g/l recipe and a 200 ml cup. Therefore your starting point is: 60 g/l × 0.2 l × 4 cups/d = 48 g/d
2. Prepare a daily plan on how do you go from 48 g/d (4 cups/d) to 24 g/d (2 cups/d). I would recommending reducing the does by 1 g of beans per day. So, if you used 48 g of beans yesterday, grind only 47 g today and 46 tomorrow etc. If headaches occur, you need to go slower. If you're drinking dark roast or if you have a headache resistant head, you can probably get away with 1.5 g/d or 2 g/d reduction rate.
3. Don't switch to another type of coffee while ramping down your intake, since the different caffeine concentration in the beans will change the daily dose. You don't know the mass of caffeine, but you do know the mass of the beans you grind. Don't introduce unknown variables. This is hard enough as it is.
4. Don't drink coffee made by other people. You won't be able to control your intake properly.

That's how I do it when ramping down my intake. You can also go all the way to zero if you like. The same logic applies to tea as well, but doing it gets a bit tricky. The concentration of caffeine in the solids is much higher in tea, so 1 g/d reduction rate is far too rapid. You also need to have a good scale, and you need to weigh very small masses, which may require some trickery.