Include the amalgamation .c and .h files in your project. Make sure that your toolchain understands that '.c' files are C, not C++.
Call the functions in the C file the same way you'd call any other functions from Unity. This depends which scripting language you're using in Unity, but if I recall correctly you use <code>extern</code> or <code>#include</code> as appropriate, then make the calls passing appropriate parameters. One consideration is parsing the results returned by, for example, a <code>SELECT</code> query.
Bleh. In checking out the above I did some web searches and found good advice. Perhaps do a web search on 'unity sqlite x' where 'x' is whatever unity scripting language you're using.