HappyBuddhist.com

I sat for a long time propped up by pillows and slowly got into sitting “properly”….it took years, but start where you are and be kind to yourself.
Sitting in seiza (on your knees) can be alot more comfortable if your hips are tight (high knees).The deal is to take the pressure off your knees by sitting on a big pillow. Having your knees below you hips allows for good alignment of your spine so that it doesn’t “collapse”. The best for that is a zafu, one of the round pillows favored by zen sitters… they really know what they are doing . Get the largest zafu you can…Dharmacrafts has good ones, mine last years before they squash down and even then I think you can get “extra stuffing” to bolster them up.
besides all that? take a good yoga class and gt a daily practice of the asanas that open the hips… that won’t help right away but over time it will.
Bottom line, get the spine erect however you can and get comfortable .