Sitali Pranayama translates to 'the cooling breath'.
Perfect for Summer Time.
Firstly, sit in a comfortable upright, seated position. Secondly, spine is nice and straight. Further more, hands gently on your knees. Finally, have your legs folded, or straight in front of you if this is too hard. Most importantly, keep your pelvis slightly tilted forward. Sit on a block or bolster to support this gentle pelvic tilt.
Meanwhile, breathe in through a tongue curled like a straw (right). Subsequently, breathing out through the nose. If you are unable to curl the tongue you can perform Sitkari instead, where you breathe through the teeth which are lightly held together or sip through your lips like a straw.
Sitali pranayama is a great way to prepare you during the hot summer months for the practice of yoga.
In conclusion, this works much like an evaporative cooling system. It cools the air before it hits your lungs, impacting the gas exchange with warm gases being exchanged for cool ones. Sitali prnanayma cleanses the system of excess pita, helping with conditions such as nausea, skin eruptions, eczema, thirst, diarrhea, fever, hyperacidity, anger, inflammation, menopausal symptoms and other signs of excess heat and toxicity. 
Please refer to this great youtube clip on how to perform this pranayama
Inhaling over the tongue, cooling the blood and exhaling through the nose.
Equal inhalation and exhalation, 1:1 breath and a minimum of 30 rounds, around 5 minutes to take effect.