В чем разница между now at last и at last ?Пожалуйста, приведите примеры.

When you have "now, at last " the emphasis is more put on the present moment, that something finally has happened at the moment (this hour, minute day)
Now, at last he managed to sign up those papers ( literally he has singed them up seconds or minutes ago)
At last he managed to sing up those papers (it could have been done any time in the recent past)
Now, at last, you have to remove the dish from the oven (emphasis that you have to do it any minute now or it might burn for example..)
At last you have to remove the dish from the oven ( It's more of an abstract advice, you are telling someone the needed steps, in order to make some kind of dish, and the last one ("at last") is the aforementioned sentence. You don't know when he/she is going to do it)