Note: this step is a one time setup:

1. Open Windows Explorer (not Internet Explorer) and create a folder called something like KS Foundation Website. Create this folder in your My Documents folder.
   
2. Launch WebMatrix and Open a Folder as Site as shown in the picture below.
   
3. Browse to the folder that you created in the first step and choose Open
4. Once the site is created you need to configure it to run with PHP 5.5. To do this you need to choose Site from the bottom-left side and then Settings from the top-left side. Click on the image below for a better picture.