Rough guide...

The plaster needs to be at just the right consistency for this to work. Too wet and it runs everywhere.
Too dry and it 'pulls' as the former is rotated, causing rough areas and gaps.

Bhe easiest way of starting the ring semmed to be to form a rough circle of dollops of plaster,
using the former over the top to consolidate the general shape. Small holes and 'thin' areas needed
extra plaster adding in order to fill out to the correct profile.