How to upgrade to Snow Leopard.

If you have a DVD, then it is very easy.
Just insert the disk, press and hold Option key.
Select DVD for boot device and follow the instructions..

But if you have only dmg image file, then there are some options.

1. Make install disk from a image file. -> you need one dual layer blank DVD media.

2. Use your HDD.

1) Start "Disk Utility"
2) In left side menu, select your hdd and right click and pick partition.
3) press + button and allocate at least 10GB for new partition.
4) In right side menu of "Disk Utility" main page, click "restore"
5) Source : set dmg image file, Destination : set new partition you created right before, Check erase destination box.
6) Reboot
7) Press and hold Option key and select new partition for boot.
8) after upgrade Snow Leopard, you can remove the new partition.