From wow.incgamers.com

How to use Soulstones

1. Right click to store your own soul or target a friendly and then right click to store their soul.
2. After yours or a friendly soul is stored in the stone a 30 minute timer starts.
3. If the person whose soul is stored (you or a friendly) dies within that time you will be given the option to 'Use Soulstone' aswell as 'Release spirit'.
4. Click 'Use Soulstone' to resurrect

Facts about Soulstones

• Require a Soul Shard to conjure
• Can have only one at a time
• Are Soulbound, so can not be traded
• Will resurrect the stored soul exactly where they died, so use with caution
• As a conjured item they will disappear if your character logs out or is disconnected for 15 minutes or more.