Pranayama is best done sitting on the floor on a folded blanket. The postures suitable are Siddhasana, Virasana, Padmasana and Baddha konasana. Any other sitting posture may be taken provided the back is kept absolutely erect from the base of the spine to the neck and perpendicular to the floor. Some types, however, may be done in a reclining position as detailed later. During practice no strain should be felt in the facial muscles, eyes and ears, or in the neck muscles, shoulders, arms, thighs and feet. The thighs and arms should be relaxed deliberately since they are unconsciously tensed during pranayama.
Iyengar, BKS. Light on Yoga, p 362
Have you noticed that swastikasana is not included in the passage above, yet it is the most common asana in which we sit for pranayama. Light o Yoga was published in 1966, and the student was expected to master Padmasana and Sidhasana. However, in the fifty years since then, it has become apparent that people are getting stiffer, regardless of age. This explains the need for sitting in swastikasana.
Incert Read the extract from Light on Yoga
Watch the 15-minute video, from the UK convention in 2009, of Geeta Iyengar demonstrating more adjustments to sit correctly for seated pranayama.
Insert Sitting.D2-1.UK2009-1
Have you noticed how Geeta Iyengar separates the watching of the demonstration and explanation from practising. Make notes when watching to gain objective knowledge, then practise to the recording, then write your reflections after gaining subjective experience.