Once you have created an OmmConsumer and connected etc, you should be able to call registerClient to request your required instruments as and when you need to do so. Our examples just happen to call registerClient in the main() method as they are simple examples.
These registerClient calls can be for individual item requests or as Batch requests -as demonstrated in example370__MarketPrice__Batch.
Each item you successfully subscribe to will be allocated a unique handle - which can be obtained from the OmmConsumerEvent which is passed into the callback handlers. You can then use this handle to close an individual item as and when required - by calling unregister with the handle. You can see this in the example 300__MarketPrice__Close.
The examples mentioned above can be found in subfolders of the ema\examples\training\consumer\series300 folder in the Elektron-SDK package.