This year I visited the Boston Cow Parade.  Here's how it went.  I had a great trip because I planned ahead.  The first thing you should do is not travel directly to Boston, there is just no parking (or the parking is very expensive and it is very congested.  Also if you are going to look at the cows you will be walking away from your car, there is no reason to have to walk all the way back when you're done. What you should do is take the local 'subway.  In Boston this is called the T.  There are many lines, I suggest you take the Green Line and pick it up at Riverside Station.  There is parking there and it only costs $3.25 for all day.  This is $25 less than the parking you'll find in Boston.  The train will cost you $3 to get to the city from this station.  You can get change at the station from machines without too much trouble.  Once you get on the train, its the D branch, take the train to the Hynes Station.  This is where your cow finding adventure begins.  You'll find that all the cows are outside or can be seen from outside.  There is one exception, the Cows at Prudential Center (a mall) are mostly inside.

My second advice is to get a map of all the cows and print it out before leaving home.  You can find a map here

My friend and I arrived at the train station at 12:30 and was at Hynes by 1:00.  In general, the cows west of  Boston Common are within one block of Boyston Street.  If they are on Newbury, then they'll be on the south side.  The farthest off of Boylston you will have to go is to the Boston Marriott where there are two cows.   My friend and I spent about 2 1/2 hours seeing the cows from west of Claredon St.  (We missed the ones at the Mariott, it was just a block or so too far, maybe you can do better.  Then we had a leisurely 90 minute lunch at the Hard Rock Cafe.  It's on Claredon St about 2 blocks south of Copley Square.  After lunch we saw 4 cows on the way to Boston Common.  WE searched around Boston Common, longer than we had to.  This will save you time, all the cows at Boston Common (there are 13 of them)  are on the outskirts of the Commons.  There are no cows in the interior to find. so circle around the Commons and you'll see them all.

Next we saw the sites of Boston, which all had cows.  We saw Faneuil Hall Marketplace  (9 Cows) Downtown Crossings (3 Cows), Christopher Columbus Park (6 cows), the Wharf and Aquarium.  Lastly we doubled back to City Hall (Government Center) where we saw 5 cows and hopped on the train back to our car.  We got on the train at about 6 PM and was in our car heading for home at about 6:30.