According to the Hadith, Zakah is paid on money/wealth that one has had for a year. I do not have money/wealth that sits for a year without adding to it or removing from it. How do I go about paying Zakah?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

An amount of money that equals 85 grams of gold (a Nisaab in Sharia) is subject to Zakah if it is possessed for one complete lunar year. The Zakah rate is 2.5 percentage.
Furthermore, any profit received during the same year even before one day of the Haul (lunar year) should be added to the capital, and then you have to count the Zakah.
But, if your property (money, articles of trade…) decreases less that Nisaab before the Haul, then you have to start and appoint a new Haul.
Allah knows best.