It is usual to sit on the floor with the buttocks on a cushion (zafu) which is placed on a square mat (zabuton) so the feet are protected. You can also use a wooden kneeling stool or kneel supported by one or more cushions.
If you can sit with the legs forward, it is ideal to place one foot on the opposite thigh in the half lotus position, changing to the other foot and leg for the next sit and so on. Or if you are supple, the full lotus position is traditional but most Westerners find this challenging. Another method is to sit crossed legged with the knees down and the feet resting on the mat. You can change the legs around for the next sit.
Begin by placing the hands on the ground in front and leaning forward, then using each hand, separate the buttocks and sit up in a stable position, ideally with the knees on the ground. Then relax everything from the waist down and raise the breast bone. To eliminate hardness from the small of the back, tuck the tailbone under while remaining erect, with the shoulders and elbows relaxed and the nose in line with the navel. The chin stays level and the jaw drops slightly. The eyes remain open looking downwards and lose their focus. Place the left hand palm upwards in the upward facing palm of the right hand and bring the thumb tips together forming a circle (the cosmic mudra).
You are now in an ideal position to check again that you are upright and relaxed. Bring your focus to your inhalation and exhalation, not interfering with their natural rhythm but allowing that rhythm to gradually absorb you.
Some people prefer to sit on a chair. Sit without touching the chair back and follow the instructions above for sitting upright, placing the feet flat on the floor with the knees hip width apart, and relaxing the legs. It is customary for sitters to wear dark, plain or black clothes which are not disturbing to others.