As a western student of the martial arts sitting in seiza can be a difficult posture to understand.
Culturally we just don’t really do it, actually probably never, and often our first experience with it comes in the martial arts.
In my own training there are really two times that we sit in such a posture- sitting on the ground with your two legs tucked under each other, spine erect, and hands on the tops of your leg.
The first is after bowing to the teacher at the start or finish of class- sometimes then getting right up to a standing posture to begin practice, or start cleaning the dojo, other times sitting in the posture for a few minutes while the teacher introduces class or takes any questions.
The second time is during some formal parts of our kata (forms) practice, that require you to sit in such a posture and receive a variety of attacks.
Both require your ability to do it- the fist out of respect and politeness, and the second so you can begin to learn the lesson of the kata.
So how do you get comfortable with doing it?
The starting point is of course to work on your personal level of flexability- streach out your legs, ankles, and back. Become a bit more limber and learn to relax.
The second step is of course to just start sitting in such a posture.
Martial arts is just massive amounts of quality practice, and such a posture (kamae) is no different!
That said, don’t plan to sit in it for an hour a day- your time is better spent dedicated to other aspects of training.
Plan and work on it over time.
Sit for a few minutes each day, often before your daily practice, and in time it will become second nature. This is the best way to do it.
Finally, if and when your legs go numb from sitting that way- and they will somewhere along the line, don’t try to get up right away as you can hurt your ankles or knees!
Better to slide off to the side, slowly extend your legs while sitting on the ground, and wait for the blood to come back in your legs…