Create partition in disk utility. Insert install disk. Press and hold C on startup. Choose partition. Done.
Unfortunately you cannot just shrink your existing Lion Partition using Disk Utility in Recovery, because the Recovery Partition itself cannot be unmounted.
One of these more annoying little "features" that come with 10.7.
You'll have to burn the InstallESD image first, then boot from that one.
I'm trying right now to do the same thing, and I will post step-by-step instructions once I'm done.