Required ENV variables:

TWITTER_CONSUMER_KEY
TWITTER_CONSUMER_SECRET
TWITTER_ACCESS_TOKEN_KEY
TWITTER_ACCESS_TOKEN_SECRET
MONGO_URI
DISCORD_BOT_TOKEN
DISCORD_CMD_PREFIX
DISCORD_BOT_OWNER_ID

A Twitter application can be created here: https://apps.twitter.com
The Twitter application only needs READ access permissions.

If you do not have a Discord Bot token you can create a Discord application here: https://discordapp.com/developers/applications/
then add a 'Bot user' to the application to retrieve the Bot's token.

The following assumes the DISCORD_CMD_PREFIX is set to !

You can add/remove Discord channels to post Twitter feeds to by running !twitter [add | remove] twitter_name from that Discord channel.
The !twitter commands do not function via DMs
Only valid Twitter accounts can be added.
Newly added Twitter feeds can take up to 5 minutes to sync and start posting to Discord.

The !twitter list command will list all the channels in a guild where Twitter feeds are being posted.
The !twitter list all command lists channels for ALL guilds and is only accessible to the bot owner.