Breath in as the dot goes up, breath out as the dot goes down.
You can change the pace using the slider.
The graphic and idea is from the Coherence Coach software by HeartMath Institute.
The HeartMath version suggests feeling a positive emotion (such as by thinking of something you appreciate). However, mindfulness research suggests simply noticing whatever feeling you have at the moment. So with this version you could try either.
You may want to add background music.
I used the sine wave formula from Tuck92: