First of all, comment out every line of /etc/apt/sources.list by placing a hash sign (`#') in front of it (if it isn't already there).

Then you need to add new "deb" lines describing the location(s) where apt will get the new packages from. You can add "deb" lines for a packages mirror on a local harddisk, and/or FTP/HTTP mirrors, and/or (a set of) CD-ROMs. `Apt' will consider all packages that can be found via any "deb" line, and use the one with the highest version number, giving priority to the first mentioned lines. (So, in case of multiple mirror locations, you'd typically first name a local harddisk, then CD-ROMs, and finally FTP/HTTP mirrors.)

When the sources.list is correct, the final step of the preparation is to update apt's internal database with the command

  apt-get update