A couple of guides here:
http://news.metaparadigma.de/osx86-install-leopard-from-dmg-image-to-your-system-61/
http://everythingapple.blogspot.co.uk/2007/11/use-powerpc-mac-to-create-bootable.html
http://www.tech-recipes.com/rx/2628/install_os_x_from_dmg_file/