Getting more comfortable sitting in 正座 (seiza)

I recommend picking up a yoga block and placing it between the legs so that it supports the butt. Start with the block in the highest position, then after you get comfortable in this height (could be a few months) then move to the medium height until comfortable, then the smallest height until you get comfortable and finally remove the block. Wishing you all the best in your training.

In The New Rules of Posture, Mary Bond suggests two modifications to seiza if this position causes strain.

1. If kneeling causes strain in your knees, put a cushion between your thighs and buttocks and sit on the cushion.

   The accompanying illustration suggests this cushion should actually be separate the buttocks from the ankles and the thighs from the calves. If your quads are tight, this will reduce the demand on them by increasing the angle at your knee joint. This is in contrast with trying to keep your legs on the ground and lean back to stretch starting from seiza, which is more demanding.

2. If the position strains your ankles and feet, place a rolled towel under your ankles.

   Similarly to 1, this will allow you to start from a less demanding position by reducing the required angle at your ankles.

The section on seiza is very meager; most of the book is not about this particular position.

Presumably if you have strain in both your knees and your ankles, then you can use the cushioning in both areas.

I do not have personal experience with these methods.

I don't know if this will help you, but I've gotten great results from foam rolling and using a tennis ball to work out trigger points in my thighs and calves. I used to get stiff, painful knees and hips from seiza, which sounds different than you, but the "magic bullet" for my knees was the tennis ball work. A lot of knee issues stem from knotted calves, which isn't super obvious. I never would've believed rolling a tennis ball on my calves would possibly help my knees, but it was literally magical. Your mileage may vary but it won't hurt to try (actually it will hurt a lot, but it's a good hurt :) ). Stretching and rolling are totally different in my experience; I can stretch my calves all day and it won't get rid of the knots that a tennis ball will. Same with quads.

The most important part is doing exercises that stretch your quads. One efficient one is this: Sit on your knees, shins against the ground (like in seiza), then lean back, trying to put your shoulders and upper back on the floor behind you. Knees and legs should remain in full contact of the floor.

If you spend part of your day in front of a laptop, find a low table and spend part of that time sitting on the floor, in seiza, legs crossed, legs streched, whatever. It will give you flexibility and improve circulation on your legs.

You can also go for having some of your meals on the low table, Japanese style.

The best thing I found was to try and sit in seiza more often. This helps to get used to the position and will also help you to find your "sweet spot". For me that is with my weight on top of my feet with the ankles pushing out.

Outside of just sitting in seiza try to stretch out quads before and after as that also helps with position. - Disclaimer my issues with seiza was tight quads.

Further investigation has yielded this video on how to practice seiza. It covers the use of pillow to help support your weight (between the legs, not underneath) and various stretches.

I also uncovered this forum post which again offers a variety of methods for practising seiza. Unfortunately both involve sitting in seiza which could constitute to "Do more of it" and neither are peer reviewed or sourced. Hope it helps though.