The Zakah should be paid after the passing of a full Hawl on a yearly basis. The commodities prepared for sale should be calculated and an inventory should be made for all the existent commodities. After determining the value of all the commodities, 2.5% of that value should be paid as Zakah. As for the money found in cash, they should be added to the commodities' value and Zakah should be paid upon all of them including cash money, balances in banks, and debts owed to you by people you are sure will repay their debts. The Zakah should be paid upon all of these capitals upon the passing of the full Hawl.
As for the past years when you did not pay the Zakah, you should pay the due Zakah calculated on all of your money and commercial commodities during all these years, as this is a debt you owe to Allah and you must pay it. It is better to undertake the task of distributing your Zakah yourself or through your agent. If the government demanded the Zakah, you must pay it to it and clear yourself. As for the workshops, the Zakah is only due on items prepared for sale. There is no Zakah due on items prepared for personal use and the profits take the same ruling as the capital concerning all things prepared for sale.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.