it might help you to look into some community mindfulness or meditation classes in your neighborhood to acquainted with it. they're often free and can give you a nice introduction to it. the thing with meditation is practice. so doing it all the time, especially when you're no in the throes of symptoms. i would practice it in the morning before getting out of bed, waiting at the doctor's office, etc.

like you said, it's really hard to meditate when your stomach is fighting you.