In short: don't control it, just know it. The Buddha's instruction: "Breathing in heavily they know: 'I'm breathing in heavily.' Breathing out heavily they know: 'I'm breathing out heavily.' Breathing in lightly they know: 'I'm breathing in lightly.'" (MN 118). If you want to calm down, let the out-breath be a little longer than the in-breath.
NaturalDon't force it
Know itLong or short
CalmLonger out-breath
The 16 steps · Counting breaths · Breathe with the circle
Questions people ask
How should I breathe during meditation?
Naturally. The Buddha's instruction is to know the breath as it is, heavy or light (MN 118), not to force it. To calm down, let the out-breath be a little longer.