Why does entering a note on a staff change the other notes? How do i get them to stay the same?

I'm trying to create the attached treble clef measure note by note. Maybe I am approaching it the wrong way because I entered the first dotted note first, but then as I tried to enter the rest of the notes, as I was entering the existing notes changed.

I'm hoping that someone has had that experience and can explain why that is happening and how to avoid it.

In entering these notes, could you outline the thought process/steps I would go through?

While I would appreciate someone doing it for me, I would more appreciate someone explaining the reasoning and the how.

Thanks