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 Kometa https://github.com/Kometa-Team/Discord-Cogs
To install a cog, use this command, replacing <cog> with the name of the cog you wish to install:
[p]cog install Kometa <cog>