Primarily, make sure downloader is loaded on the Discord bot.
[p]load downloader
Next, let's add the repository to the Discord bot.
[p]repo add elfhosted https://github.com/elfhosted/redbot-cogs
To install a cog, use this command, replacing <cog> with the name of the cog you wish to install:
[p]cog install elfhosted <cog>