The method of preparing the mate infusion varies considerably from region to region, and which method yields the finest outcome is hotly debated. However, nearly all methods have some common elements. The beverage is traditionally prepared in the same gourd recipient, also called mate or guampa in Spanish and cuia in Portuguese, in which it is drunk. The gourd is nearly filled with yerba, and hot water (typically at 70–80 °C [160–180 °F], never boiling) is added.
The most common preparation involves a careful arrangement of the yerba within the gourd before adding hot water. In this method, the gourd is first filled one-half to three-quarters of the way with yerba. Too much yerba will result in a “short” mate; conversely, too little yerba results in a “long” mate, both being considered undesirable. After that, any additional herbs (“yuyo”, in Portuguese “jujo”) may be added for either health or flavor benefits; a practice most common in Paraguay, where people acquire herbs from a local yuyera (herbalist) and use the mate as a base for their herbal infusions. When the gourd is adequately filled, the preparer typically grasps it with the full hand, covering and roughly sealing the opening with the palm. Then the mate is turned upside-down, and shaken vigorously, but briefly and with gradually decreasing force, in this inverted position. This causes the finest, most powdery particles of the yerba to settle toward the preparer’s palm and the top of the mate.
Once the yerba mate has settled, the mate is carefully brought to a near-sideways angle, with the opening tilted just slightly upward of the base. The mate is then shaken very gently with a side-to-side motion. This further settles the yerba mate inside the gourd so that the finest particles move toward the opening and the yerba is layered along one side. The largest stems and other bits create a partition between the empty space on one side of the gourd and the lopsided pile of yerba on the other.
After arranging the yerba along one side of the gourd, the mate is carefully tilted back onto its base, minimizing further disturbances of the yerba as it is re-oriented to allow consumption. Some avalanche-like settling is normal, but is not desirable. The angled mound of yerba should remain, with its powdery peak still flat and mostly level with the top of the gourd. A layer of stems along its slope will slide downward and accumulate in the space opposite the yerba (though at least a portion should remain in place).
All of this careful settling of the yerba ensures that each sip contains as little particulate matter as possible, creating a smooth-running mate. The finest particles will then be as distant as possible from the filtering end of the straw. With each draw, the smaller particles would inevitably move toward the straw, but the larger particles and stems filter much of this out. A sloped arrangement provides consistent concentration and flavor with each filling of the mate.
Now the mate is ready to receive the straw. Many people choose to pour warm water into the mate before adding the straw, while others insist that the straw is best inserted into dry yerba. Wetting the yerba by gently pouring cool water into the empty space within the gourd until the water nearly reaches the top, and then allowing it to be absorbed into the yerba before adding the straw, allows the preparer to carefully shape and “pack” the yerba's slope with the straw's filtering end, which makes the overall form of the yerba within the gourd more resilient and solid. Dry yerba, on the other hand, allows a cleaner and easier insertion of the straw, though care must be taken so as not to overly disturb the arrangement of the yerba. Such a decision is entirely a personal or cultural preference. The straw is inserted with one’s thumb on the upper end of the straw, at an angle roughly perpendicular to the slope of the yerba, so that its filtering end travels into the deepest part of the yerba and comes to rest near or against the opposite wall of the gourd. It is important that the thumb forms a seal over the end of the straw when it is being inserted, or the negative pressure produced will draw in undesirable particulates.
After the above process, the yerba may be brewed. If the straw was inserted into dry yerba, the mate must first be filled once with cool water as above, then be allowed to absorb it completely (which generally takes no more than two or three minutes). Treating the yerba with cool water before the addition of hot water is essential, as it protects the yerba mate from being scalded and from the chemical breakdown of some of its desirable nutrients. Hot water may then be added by carefully pouring it, as with the cool water before, into the cavity opposite the yerba, until it reaches almost to the top of the gourd when the yerba is fully saturated. Care should be taken to maintain the dryness of the swollen top of the yerba beside the edge of the gourd’s opening.
Once the hot water has been added, the mate is ready for drinking, and it may be refilled many times before becoming lavado (that is, washed out) and losing its flavor. When this occurs, the mound of yerba can be pushed from one side of the gourd to the other, allowing water to be added along its opposite side; this revives the mate for additional re-fillings and is called “reformar o/el mate” (reforming the mate).
found at: Wikipedia