Burn the dmg, or open the dmg but don't recover it to a other harddisk. When opening the OSInstall.mpkg, just select the right volume where you can want it to be installed! But before that, you have to Partition it yourself in Diskutility.

The idea is that you install Snow leo, from within a running normal Leo system. But offcourse you have to select a different drive to install snow leo to than the current one on which leopard is running. I'f you don't have enough harddisk, and have to do all with the same harddisk it gets cumbersome because the bootloader from snow leo may interrupt viceversa with you normal leo.