Very simple technique, used in conjunction with many practices. This is the Raja Yoga version....
(I am not covering the more advanced technique here - I don't practice it...so...)
While sitting in any comfortable meditation pose, turn the tongue back (fold it upwards and backwards) so that the under surface of the tongue touches the back portion of the soft palate on the roof of the mouth. Let it rest there, relaxing the jaws, lips, and keeping the teeth a little apart.....That's it, for now. You may read all sorts of things about getting it back into the nasal cavity etc...but not for now. It is sufficient just resting as described above. Do not strain.
If you feel discomfort, release it. Take some time to build up to being able to hold it for longer periods.
Khechari Mudra is usually practised with Ujjayi Pranayama......Khechari Mudra accentuates the pressure Ujjayi exerts on the carotid sinuses which regulate blood pressure.
Do not practice if you have tongue or mouth ulcers, or if it causes a bitter taste to be secreted.
The direct concern of this mudra is to activate the psychic, physiological and endocrine processes responsible for cellular revitalization and longevity. To practise it, simply roll the tongue backwards so that the normally lower surface touches the upper palate. Try to bring the tip as far back as possible. Hold it there for as long as possible, from half a minute to 10 or 20 minutes. If khechari mudra is correctly practised in conjunction with ujjayi over a long period of time, the tongue goes into the upper nasal orifice, in the language of Kabir Das called the 'cave of the sky' where amrit, the nectar of immortality, flows constantly.
Just below the place where Hindus used to keep the sikha-chutia, tuft of hair, there is a centre called bindu chakra. Bindu means 'drop'. This centre is so called because of a small gland there which produces a secretion that is converted into nectar. This crude secretion drops down into a refining centre in the throat called vishuddhi chakra where it is purified. By practising khechari mudra for 2 to 5 years, you will be able to stimulate these 2 important centres. Then nectar will flow, rejuvenating the body. About this Kabir has written:
"When this nectar is tasted all fears, diseases, guilt and ignorance are burned away. Then inside, you will shine like the full moonlit night."
Now when you close your eyes, what do you see? Only darkness. But if you close your eyes when the nectar is flowing, you will see only light. The whole brain, the whole consciousness will be illumined. This is the aim of practising ujjayi with khechari.