Every situation is different. That is why we advise you to always consult the website of the Dutch Tax Administration. But, we can tell you this:
No Chamber of Commerce number
If you do not have a Chamber of Commerce number, you should state your earnings under the heading 'results from other activities' on your income tax return. You may have to pay income tax and a contribution pursuant to the Health Care Insurance Act (Zvw) on this amount. For more information, see the page of the Dutch Tax Administration.
Chamber of Commerce number
If you do have a Chamber of Commerce number, then you can declare your income via Ofcorz in the same way as your other income.
Your hourly rate on Ofcorz includes VAT. You as an instructor are responsible for paying this VAT.
Ofcorz is required to pay VAT over the service fee. The service fee includes VAT. This VAT is probably deductible for you.
We will add new countries to this article soon.