So... you want to set your default storage for what? For apps.... for downloads... for other?
You can set your camera to save pictures into your SD card... but you can't set your apps to be instaled into your SD card (if your phone is not rooted).
You said that the tutorial is completely uselss for LG ( did u check that one -> http/www.tomsguide.com/faq/id-2908201/move-apps-card-android.html ? ). When you download something you can transfer it into your SD card manually. If you dont have space on your device... its not reccomended to root it!
1) Clean it once per week/month... Download Clean Master from googleplay and run it once per week. The app will clean useless files and you will be okay for sure!
2) Transfer all of your data on your SD card (remember to make BACKUP! ). Move your pictures, music, videos and all kind of data (you can use computer for that...). See the tutorial.