My Ducks come up for supper every evening around 4pm, I spend about 45 minutes in the pen with them getting my snuggles and just enjoying them.  I go back into the house and head back out to the duck pen as soon as dark hits and most times my ducks are already in their house waiting for me to shut and lock the door. Now and then they'll be having fun in the pool and waddle into their house once they see me coming to the pen.  Ducks are very easy to train. I believe it took me 5 days to train my original Pekins to go into their house at night, my other ducks since then just followed the Pekins' routine. 
Just herd the ducks by walking slowly behind them, raise your left arm if you want them to go right..your right arm if you want them to go left...after a few nights of herding them to bed they'll probably start going on their own. Or at least start going to their house once they see you coming at bedtime.   
Michelle