From the Howrah station there a lot of local trains which take you to the Belur railway Station. From the station you can either walk of take a cycle rickshaw to reach Belur Math.
Plan your trip in such a way that you spend about 2 hours at Belur Math preferably in the evening. This will include a visit to the museum, spending time in meditation, and witnessing the evening arathi at Belur Math which is a treat to the eyes.
People have suggested that taxi or bus is also a good mode of commuting from the Howrah Station but I found the train journey to be less chaotic.
From belur you can take another ferry to cross the Hoogly to reach Dakshineswar. Dakshineswar also has a railway station with trains to Sealdah and Dum Dum.
Please note that the ferry service closes at 8:00 pm.
Time table of trains between Howrah and Belur Math and ferry service between Kuthighat and Belur Math are available here [http://belurmath.org/how_to_reach_us.htm]