All of the data that is used by the game is stored under the \DATA directory. Take a look there now. You will see there a bunch of files. Quite a few of the files are unpacked - these are of no interest for now. We are interested in the ones that are packed - the ones with .BIF file extension. The BIF files are containers that hold most of the game data - this is what we want to extract. BIF files alone are useless though, we will need the accompanying KEY files as well because the BIF files are indexed by KEY files - this mumbo-jumbo is irrelevant for now; if you are interested, read all about it on the KEY BIF V1.1 format page.
At moment, there are no official tool to extract these files, so we are going to rely on home-made tools. These tools are listed on the bottom of the KEY BIF V1.1 format page, amongst the external links. Pick any one of those, and use it to extract any KEY/BIF file you want.
It does not matter which tool you use, you will have to select at least one KEY file that you want to extract. Some tools allow be more specific (like UnBIF allows extracting specific BIF files), but at the end you will have to hit an extract button. Make sure that you extract all files outside of the game directories and they may interfere with the game itself: wasting resources and confusing you with non-working modifications.
The time needed to extract the data depends on the speed of your machine and the amount of data to be extracted. I took me a little over 7 minutes to extract all data from the game - so don't be surprised.
File locations Edit
When extracting (or "unbiffing") the original files, make sure that these files are NOT copied to the Witcher\Data folder. It is advisable to create a new folder, parallel to the Data folder (I called mine Witcher\unbif) and place the extracted content there.