For an autónomo, the return gathers the income from the business (revenue minus deductible expenses) together with all other income: a salary if you are also employed, rentals, interest or gains from selling shares. The tax scale is applied to that total, and then you subtract the withholdings your clients made and the advance payments you made during the year.
The filing season usually runs from April to June of the year after the one being declared, and the result can be an additional payment or a refund depending on what you prepaid. The exact dates for each campaign are listed in the tax calendar.
The typical mistake is confirming the tax agency's draft without checking it: the draft does not really know your business activity, so you have to add your business income and expenses yourself from your books. Another slip is leaving out the advance payments already made, so you end up paying more than you should.
It is linked to the quarterly advance payments (pagos fraccionados), to the withholdings shown on your invoices and to the income and expense ledgers your net business income comes from. SLs do not file this return: they pay corporate tax, although a shareholder does declare in their own return what they receive from the company.
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