You pay your monthly installment and have a certain repayment portion included. If you have set aside additional money, you may also be able to pay it in, potentially becoming debt-free faster. It depends on the agreement in your loan contract.
Some banks offer 5 or 10% special repayments, while others may offer none at all. The amount of possible special repayment is always calculated based on the original loan amount.
Example: €100,000 loan, 5% special repayment right per annum = €5,000 possible once a year.
In some contracts, there is a minimum limit, e.g., €1,000 for a special repayment, to keep processing efforts manageable. Typically, a special repayment is possible once a year. Some banks also specify the timing.