If you are buying a separate ticket from MEL-SIN-MEL, and therefore starting and ending your OWA in SIN, then the MEL-SIN-MEL mileage does not count toward the 35k of the OWA.
The point of itineraries starting elsewhere could be any combination of the following: to save mileage and get better value from the 35k, to save on taxes/cash component, because you wish to return to your home port in the middle of the OWA (which is not allowed.)
You can start booking as soon as the first flight is released, if you wish, and then add flights as they become available. Or you can wait until several -- or all -- have been released. Your strategy will depend on how flexible you're prepared to be on the routes/dates/airlines. If you want specific flights/dates/airlines then you'll need to jump on those quickly. If you're more flexible, you may choose to wait and book the first leg all at once. When you start booking, you don't have to specify this is a OWA.