You are responsible for making sure you pay tax and labour market contributions (AM-bidrag) on your B-income. You receive B-income when you work as a freelancer or consultant, or if you get a fee for a concert performance or presentation, among other things.
Read more about receiving B-income as a freelancer etc. B-income is income where tax and labour market contributions have not been deducted from the pay before you receive the amount.
Any expenses for materials, etc., you had in connection with earning the fee are tax-deductible.