Assuming that you already have unpacked the script files please follow these step-by-step instructions.
Create a subfolder on your server (i.e. /download/ or /file_download/). Copy all script files into the new folder on your server. It is important that you upload all files, folders and sub-folders to your server. It is also important that the original file and folder structure remains intact.
All files have to be uploaded in text mode (ASCII mode) except for the binary files like images or PDF. Those files have to be transferred in binary mode. Most FTP programs have the option to set the mode to Automatic. With the mode set to Automatic you won't have to worry about it..
After you have uploaded the files you can call the URL of the script in your browser, i.e.:
This should list the demo files which allow you to test whether the script works correctly.
In order to make your own files available for download, you need to upload it to the folder /downloads/ of the script.
You can now create a link to the script and put that link on your website or send it in e-mails. That link contains the path to the index.php file of the script as well as the file name of the download file as parameter. Example:
The file "file_name.txt" could be the file you've uploaded to the /downloads/ folder of the script in step 3.
Basically, just attach a question mark and the name of your download file to the URL of the script.