Hi Syrom, sorry for late reply but I see you got an answer. I tested out Steamroller myself which DMHokie suggested, and thats a big time saver script. Lovely suggestion DMHokie. And as James is saying. This phase in your learning is a lot of back and forth until concepts and workflows get internalized. It’s like that for most of us. But It’ll all go much quicker when it starts to slowly click.
One thing, on import have you made sure to turn off “Generate Lightmap UVs”? You need to expand some options on the import dialogue to see the option.