Step 1:
http://www.virtualbox.org
Download the installer and the matching extension pack.
Run the installer, then install the extension pack by double clicking it.
At the main window, click New.
Choose a name (like OS X 10.6)
Select Mac OS X as the Type
Select Mac OS X or Mac OS X 64 bit as the Version (either will work, I use 64 bit, others like 32 bit as it can be more stable)
Click Continue
Choose the appropriate RAM; 2048 is the minimum; if you have 4GB of RAM in your Mac, use 2048; if you have less, you're not going to get decent performance even with graphics accelleration. If you have more, you can bump this up to 4GB if you're planning to use memory intensive software.
Click Continue
Click Continue (this will create a virtual 20GB HD that's a sparse image -- it grows as needed)
Click Continue (you want a VDI image).
Click Continue (you want that sparse image)
Click Create (unless you want a larger max size on your disk image, than use the slider or text entry to set the size)
You now have a virtual machine set up.
Now, if you've already got a Parallels system set up, just click Settings, click Storage, click Controller: SATA, click +(Add Hard Disk) click Choose Existing Disk and select the disk. Then Set it to be SATA port 0 (move the image you already created to some other port first).
Click OK, then click Start.
If you HAVEN'T got a system already set up with Parallels/VMWare Fusion/QEmu/etc., then skip the part about adding the new storage, insert your 10.6 install DVD, and click Start. The installer should start up.
Select Disk Utility from the menu bar when you're able to, format your new disk, then quit back to the installer, and run your installation.
Next bit's important: you're probably going to want to get an account on tonymacx86.com and download iboot + multibeast.
Follow the instructions on
http://tonymacx86.blogspot.com/2010/04/ ... -x-on.html. The reason for this is that the most recent updates for 10.6 break VirtualBox's handling of your computer's EFI.
Once you've got all this set up and are updated to the latest version of 10.6 (with security patches), you'll notice your screen resolution is stuck at 1024x768. This can be fixed; I think Multibeast might even have an install option for this if you re-run it.
http://www.macbreaker.com/2012/02/snow- ... albox.html is probably the best way to do everything I've said, and it has pictures. It's meant for non-macs, but works just as well on a Mac, with the added benefit that you get the 2d/3d acceleration that isn't available on Windows.