How to add FastDL on my server?

The FastDL option refers to a another server extra for downloads. On the dl server you just upload your CUSTOM files and your cache lua file. Don't upload the standard or basic gamefiles.

sv_downloadurl "webspace.com/your_gamefiles" in server.cfg or, if you prefer, in the startupline.

Keep in mind that your FastDL server needs a fast and stable connection and if you have enough experience, repack all your addons in one big addon and upload it.

Imho is the Steam workshop way better and uncomplicated.