There are very few people who can sit in the classical meditational asanas without previously loosening up their legs over a period of time. Those who can are usually children. For this reason it is essential that the reader regularly practises special exercises to loosen up the leg muscles and joints.
At first you may feel that your legs are too stiff to sit in any of these advanced sitting positions. This happens to everyone. Yet, with perseverance, even those people with the stiffest legs can master meditative asanas. The author speaks from experience, for when he first started practising yoga, he could not even sit in sukhasana, the cross-legged pose. But with persistent practice the legs became looser and more flexible, until he was able to sit in any of the meditative asanas with more comfort than sitting in an armchair.
The mental attitude is very important. If you start pessimistically with the belief that you will never master meditative asanas, then you have immediately defeated yourself. Be optimistic. Assume that it is only a matter of time and practice before you can sit comfortably in any of the asanas. If you believe this enough then the mind, which controls the body, will automatically begin to make physical changes.
In other words, your mind will fix a role and your body will prepare and change itself accordingly. You will find that your legs become progressively flexible over a period of weeks and months. Both regular physical loosening up exercises and mental determination are necessary.
It will be with a wonderful feeling of accomplishment and satisfaction that you will eventually sit in one of the more difficult classical meditative asanas for the first time.
You may only hold the final pose for a second or so when discomfort will oblige you to quickly unfold your legs again. Yet at this stage, you have more than passed the hallway mark towards full proficiency in the asana (called asana siddhi in Sanskrit). The aim must now be to extend the period of time in which you can maintain the final pose. This must be done slowly over a period of weeks and months. No excessive force should be utilized; all that is necessary is to increase the duration by a few seconds every day.
Eventually you should be able to sit in a meditative asana for half an hour and more, without the slightest urge to move the body. This implies that the asana is comfortable and without the slightest physical pain. It is under these circumstances that you will start to gain the most from your meditational and pranayama practices. Your awareness will more easily become one-pointed for it will not be disturbed by discomfort of the body.