What Is Tummo?
In Tibetan Buddhism, tummo is the fierce goddess of heat and passion. Tummo is found in the Mahasiddha Krishnacarya and the Hevajra Tantra texts.
Tummo, also known as inner fire meditation, is a Tibetan Buddhist meditation practice that involves the cultivation of internal heat through the visualization and breath control. Tummo is a part of tantric meditation cycles for yogic heat, developed around the concept of the female deity.
To start practicing Tummo - Inner Fire Meditation you need to receive an empowerment from a qualified Vajrayana teacher.
What is Empowerment?
There are three different vehicles in Buddhism: the Hinayana Path, The Mahayana Path and the Secret Mantra - Vajrayana Path. In order to enter the Secret Mantra - Vajrayana Path you have to receive empowerment (initiation).
Empowerment is the Vajrayana ritual which awakens the special capacity for primordial wisdom to arise in the mind of the disciple.
Empowerments can only be granted by qualified vajra masters and requires for the students receiving them to maintain the specific samaya (precepts), on the basis of the refuge and bodhisattva vows.
One can't progress on Vajrayana path without empowerment.
If you are motivated by the sincere wish to transform yourself so that you will be able to benefit others, you should receive the empowerment. It is important to have a dedicated attitude. In fact, according to Buddhist psychology, unless you dedicate yourself to others, you will never be satisfied. Instead you will be bored and lonely.
You can read more about refuge and Bodhisattva vows here and here.
The Iceman
You've probably heard about the Iceman or Wim Hof.
Win Hof is is a Dutch motivational speaker and extreme athlete that developed a special technique of being exposed to cold for a long periods of time. 
The Wim Hof Method, consists of three pillars: cold exposure, breathing techniques, and commitment. The cold exposure involves immersing oneself in cold water or taking cold showers, which Wim Hof claims can improve circulation, boost the immune system, and reduce inflammation. The breathing techniques involve a specific pattern of rapid, deep breaths that Wim Hof claims can improve oxygen levels, increase energy, and reduce stress. The commitment pillar involves developing mental discipline and making a commitment to regularly practice the other two pillars.
Wim Hof has conducted numerous scientific studies to support the effectiveness of his method and has achieved a number of impressive feats, including running a marathon in the Arctic Circle in just shorts and a pair of sandals, and breaking a world record for the longest ice bath. However, some scientists and medical professionals have raised concerns about the potential risks of The Wim Hof Method, including the risk of hypothermia and hyperventilation. It is important to consult a qualified healthcare professional before starting any new physical or mental health program.
Benefits of the Tummo Practice:
Tummo - Inner Fire Meditation has a number of physical and mental health benefits, including increased energy and focus, improved immune function, and even the ability to withstand extreme cold temperatures.
"Tummo is a way of cultivating heat, or inner fire, within the body. The heat generated during tummo practice serves to purify the body and mind, and to bring about a state of physical, emotional, and spiritual well-being."
-- Yongey Mingyur Rinpoche
How Does it Work?
Tummo exists of a combination of breathing and visualization techniques, used to enter a deep state of meditation that is used to increase a person’s "inner heat". During Tummo Meditation yogis work with their breath, bringing it to a higher level, and this changes their blood circulation.
As the flame grows, starting from below the navel and rising higher and higher, it softly penetrates each of the main chakras, spreading to all the branches and every pore of our body.
The chakras are secret doors to our being. Opening them and connecting with our Tummo fire brings us great realizations. In this way, we tap into our inner wisdom. From this great presence we can realize the true nature of our mind, fundamental wisdom, and unconditional love and compassion.
On an energetic level, the Tummo fire blazes through the channels, melting away the subtlest blockages and surging energy throughout the body.
On the level of mind, Tummo is the source of love, joy, happiness and bliss.
We naturally have this warmth within us in the area below the navel chakra, but we are not usually aware of it and we do not use it. Tummo Meditation teaches us to use it properly. This inner heat enables someone to control ones body temperature, for example keeping the body warm in cold conditions.
History of Tummo Practice:
The practice of Tummo, or inner fire meditation, has a long history within Tibetan Buddhism. It is believed to have originated with the ancient Tibetan Bon tradition, which predates Buddhism in Tibet and is based on the worship of nature and the elements. The Bon tradition holds that Tummo is one of the three primary practices for achieving physical and spiritual enlightenment, along with the practices of trekchod (cutting through solidity) and thögal (direct crossing).
Tummo is also known as the "yoga of inner heat" and is believed to be related to the Hindu practice of tapas, or asceticism. In Tibetan Buddhism, Tummo is considered an advanced practice and is typically taught to students who have already mastered the foundational practices of calm abiding (shamatha) and insight (vipashyana).
Starting Tummo
First step in Tummo - Inner Fire Meditation is bringing back your lost and taken energy. We do it thought the Nine purification breaths.
Before doing the Nine purification breaths practice, you have to adjust your meditation position.
1. Sit Cross Legged. Ideally in the full lotus position.
2. Sit with the Spine Straight with the vertebrae (back bones) balanced like a “stack of golden coins”.
3. Hands in Vajra Mudra.
4. Press the tongue to the palate, just behind the teeth.
5. Lift the shoulders with straightened arms like folded eagle’s wings
6. Tuck chin slightly in.
7. Gaze should be at tip on nose or just beyond.
The Nine purification breaths practice:
First, we generate male energy.
Close your left nostril with your right hand (your right hand in Vajra Fist). Breathe in deeply with your right nostril while imagining white compassion energy is entering your body. Hold your breath for a couple of seconds.
Close your right nostril with your left hand (your left hand in Vajra Fist). Breathe out completely with your left nostril while imagining grey energy is going away from your body with all the violence and aggression.
Do it 3 times.
Secondly, we generate female energy.
Close your right nostril with your left hand (your left hand in Vajra Fist). Breathe in deeply with your left nostril while imagining red energy of appreciation and beauty around you is entering your body. Hold your breath for a couple of seconds.
Close your left nostril with your right hand (your right hand in Vajra Fist). Breath out completely with your right nostril. Imagining dirty red color of attachment is going away from your body.
Do it 3 times.
Lastly we generate energy of both male and female.
Breathe in with both nostrils. Blue color energy which symbolizes clarity is entering your body.
Hold your breath for a second.
Breathe out - tiredness and ignorance.
Do it 3 times.
To learn more how to practice Tummo - Inner Fire Meditation please watch the video below and also find a qualified Vajrayana master to teach you.