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
Now Choose the Rom you have Downloaded From the Roms Page Then Click on Import
After that Choose Import and make Sure you Choose the .vbi file from Your Phone Storage not Your Google Drive
Then You have to Wait till It Finishes and at this Moment DO NOT CLOSE THE APP
After Importing The Rom you like All you Have to Do is to Press on the Rom and Then Your OS
Will Start Normally.