This guide will show you how to download, compile and install the nvidia-legacy-340xx-driver on Debian 12. Some people install the driver directly from sid but this will mix packages from both stable and unstable which may create issues. It's recommended to update and upgrade your system before you start.
Install these packages.
sudo apt install build-essential fakeroot devscriptsYou can manually download the source code from debian.org and go to Step 2 or continue with Step 1. Don't worry you won't be able to install packages from sid.
Add 'deb-src http://httpredir.debian.org/debian unstable main non-free contrib' to '/etc/apt/sources.list'.