The netinst CD image is a popular image which can be used to install bullseye with the debian-installer. This installation method is intended to boot from the image 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 bullseye system. If you'd rather, you can get a full size CD/DVD image which will not need the network to install. You only need the first image of such set.

Download whichever type you prefer and burn it to an optical disc.

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/DVD image to the top-level directory of the hard disk. Make sure that the image has a filename ending in .iso. Now it's just a matter of booting linux with the initrd.