First thing you should do is root your LG G3 if you want to install custom ROMs, so follow:
For rooting KitKat Android 4.4.2/4.4.3/4.4.4: How to Root LG G3 first!
For rooting Lollipop Android 5.0.1/5.0.2/5.1: How to Root LG G Flex 2, G2, and G3!
Next, if you want to install custom ROMs, you need to have a custom recovery installed, so follow How to Install CWM/TWRP Recovery on LG G3!
Before you install a custom ROM, please make sure to make a backup ROM, follow How to Backup/Restore ROM on LG G3!
Finally, to install custom ROM, download the ROM zip file you want to install then follow How to Install Custom ROM on LG G3!
If you ever end up in a nasty bootloop or simply want to return your LG G3 back to 100% stock, see How to Unroot/Unbrick LG G3!
Want to enable all apps for Dual Window? See How to Enable All Apps for Dual Window!