REST is based on HTTP so in order to call the REST SOE endpoint you simply send an HTTP request (GET or POST) to the endpoint (URI) using whatever mechanism is provided by the Flex framework.
The URI that you send the request to consists of a fixed prefix and REST SOE schema specific suffix. Use the REST Services Directory in order to understand how to form the URL. It is essentially a hierarchical scheme.
The typical response to a request is JSON.
If you want to see trivial SOE examples then look in:
C:\Program Files (x86)\ArcGIS\DeveloperKit10.1\Samples\ArcObjectsNet
I think that you should be able to find a large number of Flex examples which call HTTP services. The REST SOE is really just a REST endpoint which is called using HTTP.