With virtually zero info given about your equipment. You plug them in with the correct cables. Assuming your TV is even has compatible inputs and outputs for your old equipment. DVD, VCR, Cablebox, &c to HDTV inputs. HDTV audio output to your stereo. That way when you change inputs on your TV. The audio source to your stereo changes. So, you don't have to fiddle with both.
If you have a universal remote. Have it set to work the volume on the stereo by default (leaving the TV speakers muted). So, you don't have to fiddle with two remotes.