Late 2011, MacBook Pro and Snow Leopard

Personally, I did not do that, but from research it is possible

This article helped

Method 1: If you do not have a cooked backup of your drive

This method erases your Mac but you can make a backup:

  1. You must save the locations where your documents, e.t.c music are stored on an external drive.

  2. Then insert the Snow Leopard disc, restart your Mac and hold down the C key to start your DVD player.

  3. Once the installer appears, go to Utilities in the menu bar and click Disk Utility.

  4. Locate the drive in the sidebar where Lion is installed, select it, and go to the Partition tab.

  5. Click the drop-down menu under Partition Layout and select "1 Partition". Click Apply.

  6. Now go to the Erase tab and make sure "Mac OS Extended (Journaled)" is selected, then press the Erase button to delete all data from the screen.
    drive.

  7. Quit Disk Utility to return to the Snow Leopard installation program and continue the installation.

  8. Restore your user folder and documents from your backup.

  9. Run Software Update to get all the updates, install your applications from your original installation media, and update them as well.

Method 2: If You Have Successfully Clone Your Drive

A popular option for cloning your disc is Carbon Copy Cloner, which
is a handy utility to have for any Mac owner. Assuming you're done
follow the instructions above for your reader to be ready and ready,
but stop at step 6 and follow the instructions instead.

  1. Attach the drive containing your CCC clone to your Mac.

  2. Open System Preferences, and go to the Startup Disk preferences pane.

  3. Choose your CCC external drive as your boot disk.

  4. Restart and launch CCC, then choose your backup from the "source" menu.

  5. Select the internal drive of your Mac as the destination.

  6. Choose "Restore items from a backup" in the settings pop-up menu, and then click the "Clone" button.