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Menu Tools → Options → Package Manager

Give a name and folder location. Click OK. Drop your NuGet package files in that folder.
Go to your Project in Solution Explorer, right click and select "Manage NuGet Packages". Select your new package source.

Here is the documentation.
You can also use the Package Manager Console and invoke the Install-Package cmdlet by specifying the path to the directory that contains the package file in the -Source parameter:
Install-Package SomePackage -Source C:\PathToThePackageDir\