You want to know where to place the Visual Studio solution file when your solution contains multiple projects.
The solution file (.sln) in multiple-project solutions should be placed in its own directory, and each individual project should go in a subdirectory of the main one.
One way to accomplish this is by creating a blank solution using the File | New | Project menu. The Blank Solution template is located under the Other Project Types node in the New Project window. Next, use the Add | New Project command to add individual projects to the solution.