My sink bottom is not flat.

No problem!  The support rails and brackets have built in “play” to allow for variances in the sink bottom.   The shape of each support rail channel provide strength and two horizontal surfaces for the sink to rest on.  In some cases the sink may only rest on one horizontal surface of each support rail.  Shaws Original fireclay sinks are a great example of sinks that are not flat or “true” on the bottom and the support rails are better than flat 2x4s or plywood at adapting to the sink bottom.  Lite Weight stainless sinks (often with rims) are also a great example of sinks with bottoms that are sloped for drainage.   The play in the brackets and rails easily adapt to the sloped sink bottom.  However, the sink may sit on only one horizontal support of each support rail but this is more than enough support and far superior to supporting the sink by the rim.