Creating the Visual Studio extension project

In the following we assume the creation of a separate new tool window for Visual Studio:

create a new VSIX project within Visual Studio (New Project / Visual C# / Extensibility)
add a tool window item template (Add / New Item / Visual C# / Extensibility / Custom tool window)
the generated files can immediately be built
start of debugging causes the start of an experimental instance of Visual Studio where your extension is already installed. In the experimental instance of Visual Studio you can test out the functionality without affecting your Visual Studio installation.
your new tool window can be opened via View / Other Windows / YourWindowName
the generated VSIX file automatically installs your extension on your computer
For more detailed info and a step to step guide see Welcome to VS extensibility

Updating a Visual Studio extension at VS Marketplace

Sign in to Visual Studio Marketplace and navigate to "Manage" page. Besides the version entry of your extension click the innocent looking 3 points "...". Within the upcoming context menu select "Edit"
On the edit page simply use the first point to upload a version. The version number itself will update automatically within the web page.
See also: How to publish an update to visual studio extension