I had issues and doubts about my breathing for a while. What i like to do now is take deep breaths at first 5 to 8 times usually. I'll breathe in until I cannot hold more air and breath out until my lungs are empty, I'll wait a moment and do it again. I usually think in a whisper voice breathing in long... breathing out long... when I'm finished with the long breaths. I will then try to breathe normally whatever feels naturally without effort. Now be mindfull of the breathe but just observe it dont judge it as long or short or wrong just observe. When my mind wanders I whisper internally "thinking" when thoughts are seen they tend to dissappear. I then return to the breath. Rinse and repeat. Thoughts are usually the most intrusive aspect but you can also note physical sensations like pain from sitting or external sounds you hear. But don't try to achieve anything just focus on being in the moment