Here is how I use this technique. First, I pour myself a beer. Then, I measure out the amount of hops I would like to add to the keg and place those in the coffee press. I've done this with both whole hops and pellet with fine results, for me a lot of it is a matter of what I have on hand.
Heat enough water to cover the hops in the press to a temperature of 170f. approximately, but no hotter than that, as it may extract too much bitterness.
Add the hot water to the hops in the press and let rest for 15 to 20 minutes. I add about two cups of water and expect to lose about half of that as the hops absorb the liquid. I end up with a cup of liquid going in the keg. You will have to experiment to get the right ratio of water to hops for your desired results and you may have to play with it a few times to get it to your tastes. There are no hard and fast rules to this, you just have to mess with it.
Press the hops down as far as possible allowing the hop infused water to rise to the surface. Pour this liquid into another vessel and set in the freezer until it cools to the same temperature of the keg that you will be adding this solution. Taste to make sure it has the flavor you were expecting, you don't want to dump this in you precious beer if it isn't to your liking. Gently pour the liquid into the beer and re-seal and pressurize the keg.
I would recommend beginning with a small charge (1 oz. or less) to begin with and adding additional hops to taste or adding dry hops to augment your results. You can also use this process to add bitterness to a beer that may have turned out too sweet for you, maybe under attenuated or just out of balance. Simply heat the water to boiling temperature and leave the hops in it for an hour and add as noted above. It's easy to go too far doing this so tread gently and ramp up as needed.
If you do this, leave a comment on your technique.