Usual fermentation using a strain of yeast that will give a very dry mead (for example Champagne yeast, saccharomyces bayanus, but this is not mandatory). The alcohol content must be between 10 and 11 % (20-22 proof). If lower, the CO2 would not be absorbed and so there would be no bubbles. If higher the second fermentation may have troubles starting [V3]. It is therefore necessary to measure honey accurately and to use a reliable yeast.
When the fermentation is done, rack as usual until the mead clarifies. Take must be taken to adjust the acidity, a lack of acid would reduce the complexity of the mead [V3]. Do not SO2 when bottling because it would make the life of the yeasts harder during the second fermentation. When mead could be bottled, add yeasts, sugar and nutrients. 27 g of sugar per liter is the legal maximum in Champagne [V8]. Knowing that 4 g/L give 1 atm of pressure [V4], if too much sugar is added the pressure may be too high, which can be dangerous. V4 advises to hydrate them following maker's directions and then add some mead (less than 5 % of the batch) so that they get used to this hostile environment where they are about to go. The yeast must be a bayanus (Red Star Pasteur Champagne is not a Champagne yeast and cannot be used for the second fermentation). Get information from the maker to use the right yeast (Red Star Première cuvée or Lalvin EC-1118 for example).
Bottle immediately. As there is still sme sugar, the fermentation will restart. It is therefore mandatory to use bottles tolerating high pressure, that is Champagne bottles. In Champagne, bottles are capped with "bidule" (figure 16) to prevent any leak topped with a beer cap [V8]. "Bidule" means something like "thingy""bidule" [V8] →
Bottles are stored on their side at a low temperature (ca. 10-12°C) so that the fermentation is not too fast: the slower the fermentation, the finer the bubbles. In Champagne, the second fermentation (or prise de mousse) lasts at least 15 months (legal minimum) but can last as long as half a dozen years. A change in taste happens after 2 years on the lies and another one after 4-5 years [V9].
Lees will fall to the bottom of the bottle. To remove them, the bottle is more and more inclined (so that the neck goes down, left figure above) and "shaked" (following the scheme shown in the right figure above) so that the lees slip along the walls of the bottle and end up in the neck. This is called riddling (remuage in French). See V4 for more details about this. An alternative for those who do not want to buy or make such a rack: put bottles vertically, neck downward, in a box. Once or twice a day, raise the bottles by a few centimeters (an inch or so) and drop. The light shock is supposed to separate the yeast cells from the walls of the bottle [C. Dewitt Ward cited in V4].
Disgorging (dégorgement): store the bottles in the fridge (at a lower temperature, the pressure is also lower and the process will be less explosive) then place them upside down with the neck in a mixture a crushed ice and salt to imprison the lees in ice (figure 19). When the bottle is the opened, lees are pushed away by the pressure (the bottled should be directed toward something that will stop projectiles and avoid hazardous bounces, and do not forget to be properly protected when doing this.) Quickly clean the inside of the neck and add some "liqueur de dosage" (alcohol, sugar, 20-25 ppm SO2) to top the bottle (dosage). Depending on the amount of sugar added, the mead is brut (< 15 g/L), dry (17-35 g/L), demi-sec (33-50 g/L) or sweet (> 50 g/L). Cork immediately the bottle with a mushroom cork if you have the equipment (or else use a plastic mushroom) and tie with a metal wire. Beer caps can also be used.
In order to let the mead "se rasseoir" (which literally means "sit down"), that is forget about the trauma of dégorgement and let the liqueur mix well, the bottle is left upright for several months before aging or drinking.
References: Gifford in V4 (chapter 5), chapters 9 in V3, 11 in V6, 13 in V9, chapter 20 of V2, F3.