Invite the bot to your Discord server from the Portal's OAuth2 page.
Find your own Discord user ID: in Discord turn on Developer Mode in settings, then right-click your name and choose Copy User ID.
Choose the bot type
Open your server in the panel, stay on Overview and click Edit Server Type. Choose Red, Muse or JMusicBot. Changing the type reinstalls the server and by default deletes its files, so do this on a new server. See How to change your server type or game version.
Fill in the settings
Open Settings, then Game Settings.
Red: enter Discord Bot Token, Command Prefix and Owner. Red is a large, general bot with music, moderation and many add-ons you install from inside Discord.
Muse: enter Discord Bot Token. Muse also needs a YouTube API Key, and a Spotify Client ID and Secret if you want Spotify links. The page for each key explains how to get it.
JMusicBot: enter Discord Bot Token and Bot Owner ID, and optionally the prefix and status fields. JMusicBot is a simple music-only bot.
Start it
Open Console and start the server. The first start downloads the bot, so it takes longer. When the console shows the bot logged in, it appears online in your Discord server.
Good to know
What each bot can play depends on the bot's own project. Sites such as YouTube change often and can block bots without warning, so a source that worked last month may stop. The project's own page has the current status.
For a lot of music across many Discord servers, a separate Lavalink node is the usual approach. It is not part of these plans yet.