minor correction in the second sentence. It should be: "I saw how you were staring at that girl." These two sentences have slightly different meanings. In the first one, it is a simple statement. "I saw you staring at that girl." The speaker is stating what the other person did. In the second sentence however, the speaker is noticing "how" the other person was staring at the girl. In other words "in what way" he was staring at the girl. So, imagine you are telling someone this: "I saw how you were staring at that girl." You noticed that the other person was staring at the girl in a creepy/loving/confused/etc. way. Does this make sense? The speaker doesn't yet make it clear in what way the other was staring at the girl, but the speaker is telling the other person that they noticed that about them. The second sentence can sound like an accusation as well; as if to tell the person what they were doing was wrong, but that totally depends on the context around the situation. I hope this makes it clear. If you have further questions, please let me know.

The first one is fine, but the second sentence is missing a word. It should be, "I saw how you WERE staring at that girl" or "how you ARE staring at that girl" depending on if you want it to be past or present tense.