First of All Before Anything Make Sure you Have Enough Storage from Settings -> Storage Then Check How Much
Free Storage do you Have ? At least You Should Have Between 15GB -> 50GB Free Storage For Roms
In v2.7 Lite Version You Will Find The Rom Manager Button on the Top Right or It Will Just Open Automatically after you Sign Up
Meanwhile in v2.8 Debug Version The Button is Located in the Bottom Left
After That Click on The Custom Rom Button
After You Opened The Custom Rom Manager You Will Find a Button on The Top Right Click on It And Choose The CVBI File