First, use an applicable windows/mac device and go to the AJ classic installer website. Download the .exe file. Next, go to google drive and sign in. From there, you'll want to upload the AJ exe file onto your google drive. Once it's uploaded, head over to your steam deck. Switch to desktop mode and then open your browser and sign into your google drive. Then proceed to download the .exe file you just uploaded onto your deck.
Now that you've downloaded the .exe file, open steam on desktop mode. In the bottom left corner click "add a game", then click add a non steam game. Then you'll want to click browse in the bottom left corner and locate the AJ exe file. It should be named "AJClassicInstaller.exe". Select that file, then click open. Once it's selected click "Add selected programs". Now it will be in your steam library.
Switch back to gaming mode. Then, navigate to AJ in your library and click on it. On the right you'll see a cog wheel. Click it and go to properties. Scroll to compatibility and select "Force the use of a specific steam play compatibility tool." Then select "Proton 7.0-6". If you don't have it installed it should still show up and download for you.
If you run into any issues or don't understand anything in this guide, contact me here: steamdeckbay@gmail.com