Its not that simple because of the locked boot loader.
First of all why do you need a custom ROM on this phone? its vanilla android with a few good enhancements that you will lose with a custom ROM.
With a locked bootloader your only alternative is to use safestrap. The developer choose not to make a thread on xda. You will have to visit rootzwiki and find it. However, you should really understand the danger of having a locked bootloader and messing with safe strap and custom Roms. If something goes bad or you don't know what you're doing you might end up with a brick.
You should do a little more research before continuing. But now that you have root access xposed framework is really all you need. Combined with gravitybox there's no reason to mess with safestrap or custom Roms.
The Search bar is your best friend.
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