First, download (or otherwise obtain) the ISO disk image and burn it to a blank CD:
$ wget http://gluck.debian.org/cdimage/testing/sarge_d-i/i386/beta3/\ sarge-i386-netinst.iso $ cdrecord -data sarge-i386-netinst.isoPlace the CD into the CD drive and turn the computer on. Ensure your computer has booting from CD-ROM enabled (this is a function of the BIOS so you made need to go into the BIOS setup program at boot time). You will be asked to press Enter to boot Debian--do so.