Instructions :
Step 1
Open the info.mxi file in Notepad++ (if you don't have it then download it at https://notepad-plus-plus.org/downloads/)
Step 2
Copy the Flash Extension Template folder and rename the copy to the name of your extension.
Step 3
Read this pretty cool guide on making extensions at https://ajarproductions.com/blog/2011/02/08/creating-flash-extensions-pt1/ (only make jsfl commands), also, it requires Adobe Animate (Not Animate CC and/or Flash CC, rather the ones from Flash 8 and up at https://animatearchive.neocities.org). Make a new Flash JavaScript file (jsfl). I use Flash CS6, and Macromedia and CS versions are free due to them being abandonware) Make sure to test the .jsfl
Step 4
Once you're done, open the .mxi and rename the following :
Your Name Here = Your name!
Information goes here = The description
NAME HERE = The name of your extension.
NAME OF JSFL = The name of your .jsfl.
And of course, save, then rename info.mxi to the name of your extension.
Step 5
Open the .mxi with Macromedia Extension Manager if you want it to work with Flash 8 or Flash 9/CS3 (If you installed Flash CS3) and up (If you installed Flash 8), or Adobe Extension Manager if you want it to work with Flash 10 (Flash CS4) and up. Now click save on the popup that asks you to save the .mxp/.zxp and then try installing the mxp/zxp. If it works and you click Accept, then Congratulations! You've installed your extension! Now go to the menu Commands > Your name of the extension, and then you're done!