Installing Debian Squeeze with Long Term Support (LTS) is not recommended. But if you really want to, here is a howto.
Make sure, the conditions fit to your system (arch: i386/amd64 only, several unsupported packages, you may use backports to get recent versions). Read the FAQ.
Download the install medium from http://cdimage.debian.org/cdimage/archive/6.0.9 .
Start installing as usual.