git clone https://github.com/a11ywatch/vscode-extension.git
cd vscode-extension
npm i
After the install process you can press F5 to "Start Debugging" (or select in menu "Debug" -> "Run Extension").
A new Extension Development Host window will open in which you need to open command palette (Ctrl/Cmd + Shift + P) and select "Webview A11yWatch: Open Webview" to open application.