tasmin-sati śvāsa-praśvāsayoḥ-gati-vicchedaḥ prāṇāyāmaḥ
Having accomplished this [asana], mastery of the breath is required by controlling your inhalations and exhalations
After asana comes prāṇāyāma.
Only by practising prāṇāyāma can you expand the life (prāṇā) within you. This is done through breath regulation.
First things first; you must be able to sit still in a position of comfort (asana) for a prolonged period to practice prāṇāyāma. Hence, after asana comes prāṇāyāma. You will only build the physical discipline necessary to sit still by first practising yoga asana, and after following the strict social observances (yama-s) and social restraints (niyama-s).
For a detailed guide on the practice of prāṇāyāma see: So what is pranayama? (pranavashyauk.com)
Damian Cadman-Jones is an authorised teacher of Prana Vashya Yoga™