Have a look at this article from a yeast manufacturer.
http://www.lallemandwine.us/pdf/arti..._14-aug-02.pdf

There are several ways to go - you just need to pick one. The yeast
companies recommend a multi-step process of hydrating first with warm
water, then adding a small amount of juice (sugar) for the yeast to
begin digesting. This will yield the highest level of active, working
yeast within the first couple of hours. However, it does require a
thermometer, and some attention to following the directions carefully,
at the indicated times.

The kit manufacturers (from long experience) have found that many
customers aren't very good at following directions, may not have a
thermometer, may not remember after 10 minutes to add juice to the
rehydrated yeast, etc. They have found that simply sprinkling the
yeast on the surface of the must, covering the container loosely and
walking away will produce results nearly as good as following the yeast
companies' much more complicated instructions, at least for kit wines.
Yeast reproduces rapidly, once it is activated, so I think either
approach will work fine for home winemakers. Just don't try to combine
the methods -- if you are doing the "sprinkle and walk away" method, do
just that. Resist the temptation to stir the yeast in - that can
expose the yeast to too much sugar too quickly. Personally, I use the
"sprinkle and walk away" method for all the kits I do (40+ so far), and
have found that it works just fine. Once the yeast is activated and
visibly fermenting (bubbling, etc.), feel free to stir the must if you
feel so inclined. It won't hurt.

Doug