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
First Choose a Suitable JPG Image for Your Custom Rom If a One Didn't Work Try Another Image
Second Choose a Name For your Custom Rom Which You can Call it Whatever You Want
Third Choose The ISO File Which Could be Any ISO File(An Installation Disk or a Game Iso)
Forth Choose an Rom File like a (qcow2, vmdk, vhd, vdi or img) That you Can Download from Internet
If You Don't Have One You Can Use an Empty one From Here And Import an Windows Installation ISO in the Custom Rom
But REMEMBER Everytime you Import a Rom or a ISO File You Have To Wait OR The Custom Rom You Made Will Basically NOT WORK
Then Type The QEMU Params Which Specify Your Virtual Machine Specification (QEMU Documentation)
For Example "-vga vmware -net nic,model=e1000 -net user"
After you Finish Click on Import Then Try the New Rom You Made
If You Find an Issue Try Missing with the QEMU Params you Typed
Or Maybe Your ISO File or the Virtual Harddisk is Corrupted