Go to the main repository page. Near the top of the page, you should see a green Download Code button. Click on it, and select the Download ZIP option. This will download a file named LuckyAPI-master.zip.
Open up Steam, and find Luck be a Landlord in your Library. Right click on the game in your Library, and select Manage -> Browse Local Files.
You should be in a folder with a file called Luck be a Landlord.exe. Place the LuckyAPI-master.zip from earlier into this folder.
Next, unzip the LuckyAPI-master.zip folder using whatever software you have available. On Windows, you can do this by selecting the .zip file by clicking on it, right-clicking on the file to bring up its options, and selecting Extract All....
You should now have a folder named LuckyAPI-master in the same folder as before. Go into this folder, and go into the folder named LuckyAPI-masteragain. There should be a few files and folders here, including a folder named luckyapi. Take the luckyapi folder, and move it up two folders, so that it is with the Luck be a Landlord.exe file.
You may delete the LuckyAPI-master.zip file and the LuckyAPI-master folder if you want, but you can also leave them if you like. Next, reopen Steam, and go back to Luck be a Landlord in your Steam Library. Again, right-click on the game in your Library, but this time, select Properties....
Finally, look for the Launch Options section of the window that just popped up. In the text box, paste this text in: --script luckyapi/bootstrap.gd. Once you have done this, you've successfully installed LuckyAPI!
Go back to the main Steam window, and launch the game normally. If LuckyAPI was installed correctly, you should see the LuckyAPI version number, as well as the number of loaded mods on the main menu. If you don't see this, go back and make sure you followed all of the steps correctly! If you can't get it to work, feel free to ask for help in Our Discord Server!