She learned it in increments, baby steps if you will. Like this:
The foundation:
1. Teach the "take it, leave it" game. Show puppy you have yummy treats. Treats are dropped on the ground, one by one in different places. As you drop a treat, you say "Take it!", and of course puppy does, LOL! Good puppy! After about five repetitions of this, you drop one next to your foot and cover it with your foot and say "Leave it!" Puppy will try to get it, but won't be able to and eventually will look at you, wondering why they can't get it.......to which you say "GOOOOOOOD Leave it!!!!" and give the puppy a treat from your mouth. Teach this game well, and every time your puppy hears "Leave it" he/she will look at your face for a treat.
Generalizing it to barking at the window:
2. I'm in the room with her when she barks. I say "Leave it!", and at first may have needed to walk up and lightly tap her on the head with my finger to get her to divert her attention from the "offender" to me. Once she did, I immediately leaped in the air, threw my hands up and whooped "YES!!! GOOD LEAVE IT!" "C'mon, CHILI - let's go get a TREAT!!" and I take off running for the refrigerator and produce something delicious for her. She joyously gallops with me, because it's obvious to her something wonderful just happened, even if she hasn't totally figured it out yet.
3. This progresses to me being across the room and having her respond. Same enthusiastic response from me (which gets and keeps her attention) and same reward. Next thing I know she's running to me as soon as I give her a "Yes! GOOD Leave it!" And we race to the fridge.
4. Now I'm out of sight in the next room. Now she bolts away from the window when she hears me, and we meet at the fridge.
5. After that, piece of cake to have her gallop upstairs. If I don't meet her at the fridge, she's going to come and GET me, LOL. I keep treats upstairs, too, so I can reward her without even getting up.
I love my dog!