Hi,

Go to Setup.

Navigate to the QuickFind Box.

Type in S-Controls.

Select S-Controls.

Hit New Custom S-Control to start creating a new element.
Fill out all the required attributes of your new S-Control using code syntax.
Click on Check Syntax to let Salesforce check merge fields & functions first.
If you’re not yet satisfied and still have some editing to do, hit Quick Save and then carry on editing. Otherwise, hit Save and proceed to the next step.
Your new S-Control needs to be displayed to your users. You have several options for displaying the S-Control:
Creating a new Web tab
Creating a link or custom button
Adding the S-control to a dashboard
Adding the S-Control to a page layout
You can edit an existing S-Control by clicking on Edit instead of New Custom S-Control on the S-Controls dialog box.

S-Controls Wrap-up
An S-Control is a flexible and customizable way to add useful elements to your Salesforce organization interface. You simply have to create the proper code syntax for your elements to act as you want them to be. S-Controls can easily be edited as well.

Again, the S-Control feature is already defunct for new users. But it is still functional in old org users before the 2009 Salesforce roll-out.

Thanks.