If you have partitioned your internal HDD, if that is what you mean with "
installed + partitioned OS X Lion" and installed Lion onto the second partition and leaving Mac OS X 10.6 Snow Leopard intact, you can just boot back via holding down the OPTION key during startup or using System Preferences > Startup Disk and selecting the SL partition. Then you can use Disk Utility to remove the Lion partition and finished your are.
Or have you installed Lion over SL, onto the same partition as SL?
Then:
Short version:
Clean Install of OS X 10.6 Snow Leopard
Long version:
Mac OS X v10.6 Snow Leopard: How to Erase and Install