Redeem a code
A creator gave you a code, usually as a link in a chat. It gives your account one game, and the page you redeem it on takes you all the way to installing it.
Open the link, or type the code
A redeem link opens straight on the game. If you were given only the code, go to boardflight.app/redeem and type it. It is three groups of four, like WXYZ-4821-QRST. Capitals and dashes do not matter.
Look at what you are being given
The page shows the game, who is sharing it and which version you would get, before it asks you for anything.
Sign in, or make an account
A game is given to an account, not to a device. That is what lets you come back for it, or for a newer version, from any browser. If you have no account, the page makes one right there.
Redeem it
Press Redeem. The game is in My Games from then on. Whoever gave you the code can take the game back.
Put it on your Board
The same page then offers Install. With BoardFlight on your computer and your Board paired, it puts the game on your Board in one press. Get BoardFlight
If the code is not accepted
The page says only that the code does not exist or has been revoked. Check it against what you were sent. A code can be limited to a number of uses or to a date, so if it still fails, ask whoever sent it for a new one.
Given access by email instead of a code? Sign in and look in My Games. It will be there.