How specific does customs form exactly have to be?

I'm returning to USA soon from Japan. I never filled out a customs form. I did some research but just need clarification. Do I need to be super specific and list everything like "Women's puffer jacket", "Women's t-shirts", etc? And their exact value? I have way too many snacks, postcards, gachapons, figurines, game merchandise too. Can someone give me advice and example on how I should declare my items when filling out customs form?

Also, is the customs form given at plane or when I arrive to USA?

Thank you!