When a player joins without one of your custom maps or mods, they download it automatically. Your server has built-in fast downloads (FastDL), so there is nothing to set up: every custom map and mod .iwd file you upload (except names starting with iw_ or localized_) is offered to players over a fast web download when the server starts.
Note
New or changed files are offered to players after the next restart. The game's own files (iw_ and localized_ files) are never downloadable, so give custom files other names.