Fortunately iD makes it really easy to create them. Here's how:
- Draw the OUTER area and tag it appropriately. In this case, leisure=park.
- Using the "line" or "area" tool, draw an INNER area. Or, select another existing feature inside the one you just drew.
- Select the outer and inner features, and press C. This will create a multipolygon automatically.
- Just to double check, you should now have a relation with both features as members. One with the inner role, and one with the outer role.
Let me know if you have any questions.