An easy way...
It is more intuitive and therefor perhaps safer to use a partitioning tool with a GUI.
http://partedmagic.com/
You should always have a backup of any files you don't want to loose and make a notation of your partition table before you begin.
However, merely shrinking a partition should not be dire for the partition involved or its contents. Powerful partitioning tools with a CLI that are mishandled are a real danger.