1. You need the image (iso/dmg) from the DVD. Finally you need the *.dmg. If you have the iso, convert it with the disk utility.
2. Open disk utility > click your hard drive > click the partitions tab > press the plus sign underneath the box showing your current partition > select the size of the new partition (do this twice, one partition with about 7GB for the dmg, the other about 15GB for the install)
3. Put the snow leopard image (dmg) onto the 7GB partition.
4. Open disk utility > click your hard drive > click restore tab > drag & drop the image from wherever it is to the source > drag & drop the 15GB partition to the destination > click restore.
5. Go to system preferences and change your startup disk to the partition with the dmg on it. Restart your computer and enjoy your virtual dvdrom-drive.
6. Before the installation, open the disk utility and delete your (current) partition, (which will probably be the biggest one) and make it new.
7. Now, to install snow leopard, just choose the new (big) partition to install.
8. After you’ve installed snow Leopard go into disk utility and clean up the partitions you now no longer need (the 7GB and the 15GB).
9. That’s it!