At the end of the year, you go into SAT (Mex IRS) page to file your taxes and they provide you with a list of your money Input and Output, and they tell you how much you owe in Taxes or how much you are owed by them.
You still can add other things that were missed by the system.
You have to have your books in order anyway, and tax filing is usually as simple as copying over some numbers from the bookkeeping software and then Skatteverket (Swedish IRS) will suggest what is probably the best deductions and periodizations available for you. For my one man consulting business it takes less than an hour, and requires nothing more than having my accounting in order.
But if you are self employed, you have to hire an accountant. It is not a legal obligation but there are so many things that can go wrong that you are putting your business at risk if you don't.