The netinst CD image is a popular image which can be used to install stretch with the debian-installer. This image is intended to boot from CD and install additional packages over a network; hence the name 'netinst'. The image has the software components needed to run the installer and the base packages to provide a minimal stretch system. If you'd rather, you can get a full size CD image which will not need the network to install. You only need the first CD of the set.

Download whichever type you prefer and burn it to a CD.

It's also possible to boot debian-installer completely from the net. The various methods to netboot depend on your architecture and netboot setup. The files in netboot/ can be used to netboot debian-installer.

It's possible to boot the installer using no removable media, but just an existing hard disk, which can have a different OS on it. Download hd-media/initrd.gz, hd-media/vmlinuz, and a Debian CD image to the top-level directory of the hard disk. Make sure that the CD image has a filename ending in .iso. Now it's just a matter of booting linux with the initrd.