Hey there! Welcome to the wonderful world of TTS :D
It's pretty simple really, all you have to do is install Tabletop Simulator. When it's done, you simply need to go to a workshop mod page, like this one, and click on the big Subscribe button. Steam should initiate a download.

When all your downloads are done, launch the game (I strongly suggest playing the tutorial), then you can either create a Solo or a Multiplayer room (or join someone else's). When you're in the room, you'll be prompted with the option to load from one of your workshop files. Clicking on one will open it and you will start to download the assets (pictures and models). When everything is done downloading you're good to go!

One thing to keep in mind, Tabletop Simulator does not have an AI, it's exactly like having the board game on a table irl, you will usually have to deal with the setup, cards dealing and so on yourself.

But there are some mods that come with a bunch of scripts, like this one, to automate some parts of the game (in this case you can automatically setup the ancient ones for instance).

Hope that helped!

"Steam should initiate a download."

If Steam does not initiate a download, is there anything else that can be tried?

Do your friends need to have TTS to join a game with you?