Open the Steam app on your computer and select LIBRARY from the top menu, then select  >PLAY to launch Tabletop Simulator

Click the Play button in the popup menu. If there's an update for TTS, you will see it load here in the upper left corner of the Steam window.

If you want to see available Cosmic Encounter games in progress, click Join. If you want to start your own game, click Create.

All current games are listed. Click Search to enter Cosmic. This will filter for current Cosmic Encounter games.

All current Cosmic games are listed. Games with a lock require a password, which you'll need to get from the host. You can look them up on the Steam app, and try to get them on your friends list. You can join games without locks.

To add friends on Steam, open the app and click Community. Then enter the Host name you see in TTS. Then add the friend you saw playing Cosmic. Then you can pre-arrange to play games with them.

Click Create if you want to host your own server. Then click multiplayer so others can join you.

Set your server name, pw, and number of players. Then click Create Server.

Choose DLC (DownLoadable Content)

Choose Cosmic Encounter to launch the game. All DLCs you own will be unlocked. You can also use the search box to find what you want. 

Your game is ready! If you made a private server, send the password to your friends. They will only need to own TTS to join you. All expansion sets you purchased will be on the side tables. Try using Discord or Skype to talk with people. The built-in TTS voice has occasional glitches.