A .smil file is generally used to describe multiple streams which are keyframe aligned for adaptive bitrate streaming, you could use a .smil file for a single bitrate stream but I don't see any benefit to doing so.
An example .smil file can be found in the Adaptive bitrate streaming section of this live streaming tutorial
I've posted an example here,
The information in the .smil file needs to be as accurate as possible as it's used for switching.
<video src="myStream_700" system-bitrate="700000"/>
<video src="myStream_350" system-bitrate="350000"/>
<video src="myStream_200" system-bitrate="200000"/>