When doing pranayama you can either lie or sit down.
Either way, your position needs to be comfortable as you will be in this position for 20-25 minutes, and as you develop your pranayama this will increase to 1 hour.
If you lie down, bring your feet towards your hips so your knees are bent – feet are flat on the floor and about a 30 cm apart. Arms can be by your sides, or over the top of your head, resting on the floor
There are various options for sitting. You can sit in lotus, ½ lotus, or cross-legged on the floor. Here, your knees should be below your hips and so it may be necessary to use a cushion or block to raise your hips. If necessary, you can rest against a wall.
You can also simply sit on a firm chair. Try to have your back supporting itself and not resting against the back of the chair. You can also use a stool. Do not sit on a soft chair when doing pranayama as your back will not be straight.
Whatever position you choose, ensure that your back and neck are naturally straight but also relaxed.