How the back up your contacts on iCloud on the iPhone 4S?

Step by step instructions please?

Not possible

Step 1: Access iCloud service through the "Settings" menu on your phone and tap on "iCloud".
Step 2: Verify the iCloud account is set up and that "Contacts" is turned on.* You must enable the backup option in order for this to take effect.
Step 3: Scroll down and click "Storage & Backup" from the bottom of the interface.
Step 4: Set "iCloud Backup" toggle to On before selecting the "Back Up Now" option. Please note that all iCloud backups require a Wi-Fi connection. You can back up manually whenever your iPhone is connected to the Internet over Wi-Fi by choosing Back Up Now from Settings > iCloud > Storage & Backup.

A little complicated. Maybe you can try Syncios, a freeware to backup data including contacts on iPhone freely.