The OnePlus X and the Honor Huawei 5X have a lot in common: relatively low prices, slick case designs, and far-reaching marketing campaigns. And now both of them can be loaded with the fan-favorite Team Win Recovery Project, a custom Android recovery that makes loading custom ROMs and other modifications easy. Users can download the recovery images at the official TWRP site, here and here.
The Android community (or at least that part of it that uses custom recoveries) tends to gravitate around certain models, if only because there are so many these days that it's impossible to support every one with every ROM. The OnePlus X and Honor 5X are two such devices, often picked because their mid-range hardware are both excellent values. The fact that they're both good-looking phones doesn't hurt, either. The Honor 5X hasn't been around long enough to get any serious traction for custom ROMs just yet, but it shouldn't be too long in coming.
If you've already rooted your phone, you can install TWRP via the official app. If not, grab the images from the web and install them via fastboot with your PC. You know the drill.