>Dry yeast should be re-hydrated in water before pitching. Often the concentration of sugars in wort is high enough that the yeast can not draw enough water across the cell membranes to restart their metabolism.
According to yeast and nutrient manufacturer Lallemand, rehydration is one of the most important steps in ensuring a healthy fermentation. Rehydrating your yeast properly results in a larger, healthier population than pitching them directly into the must.
BKYeast has a rehydration article, which includes data. The yeast rehydrated in warm water fared the best. Yeast rehydrated in beer wort (chemically similar to our musts) performed terribly. Water temperature mattered significantly as well.
Different strains may require different rehydration temperatures, so check the manufacturer's website first. Most yeast is rehydrated at 104°F/40°C. If not listed - assume above temperature.
Leaving your yeast to rehydrate for longer than 15-30 minutes without attemperating or pitching is not beneficial - it will in fact begin to harm them.
Specially formulated nutrients can be added to the rehydration water. Absorbing the nutrient-rich water starts yeast on the right foot. Some nutrients are specifically unsuitable for this task, however. Diammonium Phosphate (DAP), for instance, can chemically burn the yeast at this stage. GoFerm is the most frequently mentioned rehydration nutrient on /r/mead.
Using inappropriate nutrients (anything not GoFerm) can be actively harmful as it is difficult to accurately measure very small amounts. Additionally, nutrition/nitrogen/sugar is not actively needed, water is. Yeast has imperfect membranes during rehydration and can absorb too much of a chemical which kills them.
Adding some of your must to your rehydrated yeast (after they have been rehydrated for 15-30 mins) is a good practice. This will help to acclimate the still-warm yeast to the cool must and avoid cold shock. If the difference of the temperatures of the yeast and the must are more than 10° F five minutes after initial attemperation, the process should be repeated.
Sanitize your equipment.
Calculate how much Go-Ferm and water you will use based upon how many grams of yeast you are pitching with the following forumlae:
Grams of yeast x 1.25 = grams of Go-Ferm to use
[grams of Go-Ferm] x 20 = ml of water to use.
* Example: If you are pitching 10 g of yeast, you need 12.5 g of Go-Ferm and 250 g of water.
If using tap water, boil the water to sterilize it and wait for it to cool down to 110° F. If using bottled water (or you trust your tap water), bring it to 110F on the stove or in the microwave.
Add Go-Ferm and mix thoroughly.
Sprinkle the yeast over the mixture of water and Go-Ferm. Avoid clumping.
Let it stand for 15-30 mins.
Add an equal amount of must to the vessel. After five minutes, if the difference of the temperatures of the yeast slurry and the must are more than 10° F, repeat this step.
Pitch yeast into your fermentation vessel.
Sanitize equipment. Boil water if necessary.
Heat (or allow to cool) 20mL time the amount of grams of yeast (e.g. a 5g packet would need 100mL of water) to 104F/40C.
Open and add yeast to the water. Avoid clumping.
Let stand for 15-30 minutes.
Add an equal amount of must to the vessel. After five minutes, if the difference of the temperatures of the yeast slurry and the must are more than 10° F, repeat this step.
Pitch yeast into your fermentation vessel.