Instructions
Select the modification you would like to apply. You can find lists at the download sites, like racesimulations.com (see Resources).
Download the modification you want. The files sizes vary widely from a few megabytes to over 300 megabytes, so leave plenty of time and bandwidth if you are going to have to download a large file.
Open and unpack the downloaded file. Usually you will need to use WinZip or another unpacking program to do so. Some modifications come with an executable file that will take you through an installation procedure, similar to the one for the original game. If there is an executable file, then double-click on it and follow the onscreen instructions for the rest of the installation.
Open the Read Me file if there is no executable file. This will show you how to install the modification. Usually you will need to copy the files from the modification into your "F1 Challenge 99-02" directory.
Launch the game and enjoy the new modification. Some mods will have a separate launch icon on the desktop, and others may launch via the regular icon. Check the Read Me file for more instructions.