Step 1

Go to the TaleSpire Steam page and download the "Guest Edition" of the game. 
Game Photo

Step 2

Ask your GM for an invite code to the campaign.
Game Photo

Step 3

You can redeem the invite by entering it in the "Join Campaign" box inside TaleSpire and clicking the "Join" button.

If the campaign has a seat available, you will get in immediately.

If not, the people inside the campaign will be notified that you are waiting for a seat at the table. You will enter the campaign automatically when a free seat is available.
Game Photo

Step 4

Have fun!