Brad's Iyengar Yoga Notebook
Virasana is a good pose to rest in, especially after the standing poses.   Eventually this is a resting pose, not a stretching pose.   Most people like to have a folded blanket spread across their mat for this pose so there is not so much pressure on the knees and tops of the feet.
If you feel you are sitting more in your knees and not on your sitting bones, then use a block, sandbag, or some blankets under your buttocks to take your weight onto your sitting bones and out of your knees.
Actions of the legs and feet
To facilitate coming into the asana:
(1) Come up on your knees and flatten your calves manually with your hands and roll their skin and muscles out to the sides as you sit back onto the floor or your support.   It helps to begin this action with your fingertips right into the flesh directly behind your knees.
(2) After sitting back, rotate the tops of your thighs outward manually.   If your thighs are large or have a lot of flesh, you may need to pull some of flesh of the bottom of your thighs (the hamstring area) inwardly off your calves.
(3) Remain seated and release the skin at the front of your knees by raising each knee one at a time with your hands.
(4) Take the skin and bones of your buttocks back and outward on the floor or your support.
Keep your knees together and your feet parallel.   Do not let your feet turn out to the sides or turn inward like a "sickle" shape.   Turn your inner heels toward your outer heels.   Try to be on the center of each foot.   Actively lengthen out through all your toes straight back and spread them slightly.   Press the outsides of the tops of your feet (little toe sides) to the floor as much as the insides (great toe sides).   Probably this means you need to press your little toes down more and turn your inner heels upward toward the ceiling.   Your heels should be touching the sides of your hips.   Your inner calves should be touching your outer thighs.   Turn your calves out to press your outer shinbones down.   Note in this pose that you can slide your hands between your feet and your outer hips -- do not have your feet tucked under your buttocks as in Vajrasana.   In Virasana, you sit between your feet.
The thighs have a tendency to fall inward in this pose.   Revolve your thighs and knees outward enough so that your thighs face the ceiling and your shins are perpendicular to the floor.   You want your inner knees to lift and roll outward (the inner knee rolling toward the outer knee).   The action to lift the inner knees is to press your outer shins and little toes downward to balance the thighs' tendency to roll inward.   Feel as though your thighs are as heavy as possible, sinking toward the floor, weighting your buttock bones down firmly into the floor if possible.
Establish Tadasana in your torso.   Lift your side ribs and torso up out of your pelvis.   Raise the top of your sternum upward away from your pubic bone strongly.   The idea behind lifting your sternum toward the ceiling and lengthening the front of your torso is not to make you look good, but to give you deep space in your torso in which to breathe.   Lengthen the line up from your sacrum through the crown of your head while you are continuing to ground through your buttock bones.   Also take care to establish Tadasana in your pelvis, so your sacrum and coccyx are moving under and up, not back, even though you have drawn your buttock bones and flesh back and apart on the floor or sitting support.
Actions of the hands, arms, and shoulders
Draw both your shoulder blades into and down your back to assist in expanding your chest.   Rotate both arms outward and place your palms on the soles of your feet, also to assist you in opening your chest.
To come out of the pose, come up to your knees, cross your legs behind you as if to come into Sukhasana and then sit back through Sukhasana into Dandasana, extending your legs straight out in front of you.   It's important to do something to straighten your knees for a few moments after the pose to "reset" them after having been bent so deeply.