All of the following commands should be executed in the folder containing the project from which you want to create a package.
nuget spec – Create a .nuspec file which contains meta-data about the package such as author, description, and title. You will need to edit some of the values in this file when you first create it.
nuget pack <.csproj file name> -Prop Configuration=Release – Create a .nupkg file containing the compiled contents of the project specified in the first parameter. Optionally target a specific configuration.
nuget push <.nupkg file name> – Push the NuGet package up to your NuGet.org account.
More complete documentation here.