"Now I put the sugar in the cup" means that what you're doing at that moment is part of a general procedure. For example, if you're showing someone how to make a cake and you're putting sugar into a cup, saying "Now I put sugar in the cup" means "Every time I get to this stage (of the cooking process), I put sugar into the cup". "Now I'm putting sugar into the cup" is simply a description of what you're doing at that moment.

Don't get confused by the word "now". In the first case the action occurs entirely at the time you're talking as an atomic event. In the second one the action is getting completed as you talk (equivalent to the progressive aspect of Mandarin).