How to practice your new thought

I have just finished your how to feel better course, it was wonderful, thank you.
I have a question that may be very obvious but will ask anyway!
When you have a new thought you want to think, and even if it is a neutral thought rather than going straight to a great thought. How do you practice this new thought? Do you just repeat it to yourself as often as possible to train the brain? And when the “old thought” keeps popping up do you acknowledge and feel it still or instead practice your new thought? Im guessing its normal for your old thought to keep wanting to come up and although i understand you dont want to push it away at some point you need to insert your new thought in so you can feel differently?? Thanks in advance. Kate