The first thing we need to know is what is going on with your stomach. Pranayama when done correctly can hava theraputic results. As far as Asana work any Hatha yoga type work should be done with the breath in mind which is why a teacher or an advanced and experienced yogi should be sought. You absolutely do not want to create incorrect breathing habits.
There are a number of excellent websites with information, step by step guide lines and video instructions (which are all extremely helpful) nothing beats having a live person there to observe your practice and point out where you need improvement.
A good place to start your breathing practice is by being mindful of it throughout your day. Are you breathing from the abdominal cavity, are you forcing the abdominal cavity to rise or is it naturally rising? Are you breathing from the Thoracic cavity? Are you getting full breaths? Etc…
This is a good place to start because if you do it everyday you will become very aware of your breath if you go a step further and integrate this into your Yogic practice you will become even more aware of your breath and how it works with the body and the effect it has on your mind.
Other mindful questions you can ask yourself is When you are angry or happy is your breath shallow, fast and comming from the thoracic cavity or are you breathing from your abdominal cavity? Our emotion and behavior are influenced by our breath.
Another question depending how deep you want to focus is to see if you can notice on what breath you fall asleep and on what breath you awaken. Is it on the in breath or the out breath.
An easy exercise you can do is to lie down in Sivasana (corps pose) and do the following.
Fist lie there for a couple of minutes noticing your breath. Don’t try to control it, just observe it.
When your ready it will be time to start.
Start on an in-breath from the lower abdominals. Imagine you are filling the area starting from your bladder going up into your lower abdominals with air let your tummy rise gently. Focus on the movement and feeling of the breath in that area.
Do it slowly and comfortably then let the movement of the incomming breath fill the middle of your torso feel the diaphragm engage and begin to rise gently. Next feel the flow of air rise into your chest and let it expand from side to side and bottom to top right up to your throat.
Do all these steps in one slow smooth breath. Not forced or hurried. You should not feel discomfort while doing this.
Now on the out breath you are going to feel the breath slowly moving out of your body through the nose. First feel the exhalation releasing in your throat moving down into your thoracic cavity, observing your chest lowering down into your diaphragmatic area. From there feel the exhalation release from your abdomin pulling your navel area gently towards your spine and beyond that gently pull your bladder area in towards the center of your pelvic bowl.
All this should be done smoothly, comfortably with an even flow. Just observe the process. If any thoughts should interrupt your focus that’s ok, just bring it back to the breath.
As far as the rhythm of the breath it would be good if you can try to keep the exhalations a little longer than the inhalations, but more importantly is that you observe your body and breath during the process.
Try this for 3 minutes to start and work your way up to 7 or 11 or even more. When you get familiar with this exercise you can then try it while sitting in easy pose (legs crossed) on a pillow or cussion.
Once you get familiar with both styles you can work on hold your breath for 5 seconds between inhalation - exhalation - inhalation - etc… You could try a 5 second inhale, hold breath for 5 seconds, and an 8 to 10 second exhale. When you become proficient in this way you can increase the times for as long as is comfortable for you.
I can inhale for about 15 to 20 seconds, hold breath for about 10 seconds and exhale for about 30 to 40 seconds comfortably and feel wonderful thereafter. I can push my limits on this, but that would change the intent of the exercise. Here my intention is simply to focus on the mechanism of the breath to increase breath awareness and performance within a relaxation theme. Often this is one of my first breath exercises before going onto more strenous breath exercises.
Try to encorporate this breath exercise into your practice. It will help you become aware of your breath and to relax. When you do Asana pay attention to your breath. Pay close attention to where the resistance is. Breath through it. The thing is that different asanas require different breath patterns which is why it’s a good idea to find someone who can guide you.