If you look on the packet it should have directions. If not here is info from Lalvin's website.
Importance of rehydrating yeast
......................................
How important is it to rehydrate the yeast before
adding
it to the fermenter?
Very important! Extensive research shows that the
yeast
cell wall is very fragile during the first few minutes of
rehydration.
Some of the components of the yeast are going from a dry
crystalline form
to a gel like state and can go through this transformation
successfully
if rehydrated properly (refer to our 3 easy steps rehydration
protocol).
With rehydration water at lower temperatures the transformation
from crystalline
to gel is less successful, the cell wall becomes porous and
leaches out
vital parts of its insides. Rehydration in 60°F water can result
in
a loss of 60% of the yeast viability.
Rehydration in distilled or deionized water is
lethal to
the yeast. The cell walls require the presence of some minerals,
sodium,
calcium, magnesium and or potassium, during rehydration. Tap
water at
250 ppm hardness is optimum. Most tap water has enough hardness
to do
the job. The presence of 1/2% yeast extract, yeast hulls,
autolyzed yeast
or peptone in the rehydration water will give the yeast an added
boost
that will get it through its lag phase quicker. After the yeast
cell wall
has been reconstituted, the yeast returns to its normal ability
to be
selective.
Most fermentations should start with an
inoculation of 3
- 4 million viable yeast cells per milliliter of must. A normal
healthy
fermentation will reach the stationary phase with a cell
population up
to 100 - 150 million viable yeast cells per milliliter. Because
of this
significant increase in biomass, it is critical that the active
dried
yeast gets off to a good start. Winemakers that carefully
rehydrate their
active dried yeast are taking a key preventative step to
avoiding stuck
and sluggish fermentations. Remember to use only clean 104°F tap
water
and please refer to our Easy 3 Steps Protocol for more
information.
http://www.lalvinyeast.com/strains.asp
http://www.lalvinyeast.com/index.asp