I am assuming here you know what a RAW file is so I will skip that bit. Note you can do the same with jpeg files.
Set your camera up in AV mode set F11 iso 100. (there are good reasons for this)
Take a series of shots overlapping each by about 20% left to right.
Transfer you files to your PC
In CS3 File Menu -> Automate -> Photo Merge
Select the files for the merge hit ok.
Depending on how much grunt your PC has the next bit can take a while. this one took 15sec on my PC. CS3 will automate the merge process and produce a joined file that you will have to crop to clean up the edges.
Avoid shooting from dark to light try and keep the light even across the shots.