Okay here is how you do it.
First(if you haven't already), install the GTA SA apk file and select done, do not open it yet.
Second, create the obb folder that GTA SA's obb file would normally put into. That folder is named "com.rockstargames.gtasa" and it goes in sdcard0/Android/obb.
Third, create the same folder and put in the Android/obb folder on your external card. If you don't see it, create it with that exact spelling. Make sure the "Android" folder itself is in the root directory of your external SD card.
Fourth, place the GTA SA obb file(and any other files that are needed) in the "com.rockstargames.gtasa" folder you created in your external SD card. As a reminder, that should be located in the Android/obb folder in your external sd card's root directory.
Fifth, launch folder mount(and only use this app to do this), tap the "+" icon at the top right corner of the screen. Name the game GTA SA(or whatever you want), then tap on(or right below) "Source" and select the "com.rockstar..." folder in the internal obb folder and tap the check mark icon to the right of the folder+ icon at the top right corner of the screen. Folder mount will ask if you want it to automatically create the destination folder. Answer yes. Then tap the "+" icon. Right after that, tap the thumb tack icon right next "GTA SA"(Or whatever you named it). That thumb tack icon should be green, signalling that both folders are now linked. Close foldermount and you're done!
Lastly, launch the GTA SA app and it should load the game itself. If you have anymore questions or issues, feel free to ask. Once you get the hang of it, you can use this method to install any large app from the external SD card.