I assume you have a fresh Squeak image, well packaged with Seaside already installed.

Open a Squeak Workspace and type the following code:
WAKom startOn: 9090.
 WAQCM registerAsApplication: 'the-name-of-your-seaside-application'
 Then select the above code in the Workspace and click on the 'do it' menu item

The first line of code launch the Seaside web server.

The second line of code registers your-seaside-application. This line may be omitted if your seaside application is directly registred in your seaside application code (in the initialize method).
initialize
 self registerAsApplication: 'the-name-of-your-seaside-application'
Then, open a web browser with the URL: http://localhost:9090/seaside/the-name-of-your-seaside-application

Enjoy !