Question on the model and what to work on

I am wanting to quit smoking and lose about 15 pounds. I read where it is best to work on one thing at a time. I didn’t know if I need to do these two things at different times or I could include them as working on my health at once.

Also, I was wondering about the model.
When I do it each day, do I do it on the same subject (like regarding my health) or all different ones?
Do I just keep repeating the new thought if the negative thought comes up?
It says to work on one thing at a time, does that mean I need to only work on the model regarding what I am working on at that time?

Thank you!