In searching DejaNews I found several posts on making soy yogurt. Seems it
is made like dairy yogurt. This was the best article I found:

Subject:   Re: yogurt making quandry
From:      [log in to unmask] (Tom Molnar)
Date:      1996/02/28
Newsgroup: rec.food.veg.cooking

The recent post on cow's milk yogurt reminded me that I used to make
soy yogurt.  Maybe some of you would like to try it just for fun.  It
always used to work out reasonably well for me.

It tastes different, but so what?  To me it wasn't a replacement for
regular yogurt, it was a different way to eat soy milk and a healthy one
at that!

A few things I learned about soy yogurt making was:

        + it's easier than making cow's milk yogurt
        + use plain regular fat soymilk (I used EdenSoy Original)
        + no need to boil soymilk like you do milk.
        + no need to worry unduly about initial temperatures, but the
          soymilk should be near room temperature to start.

        Starter:

        + you can use a Tablespoon of regular yogurt as a starter
          SO LONG AS the culture is alive (look for the phrase
          "live culture" on the box), or
        + add a teaspoon of non-dairy acidophilus (the Solgar brand is vegan)
          and mix.

You can just mix the starter and the soymilk and pour it into the yogurt
maker cups and let it reach the appropriate temperature.  Save some yogurt
to start your next batch.  The longer you leave it, the more tart the
flavor, and the older the starter (that is the number of times you recycle
it) the sharper the flavor.  I usually like to stop my yogurt as soon as
the yogurt gets firm (usually 7 - 8 hours).

You don't need a yogurt maker if you have an oven that can reach
roughly 100 - 105F (40 - 45C Max).  My oven was had a pilot light that was
always on and was always that temperature when the oven was off (the
pilot stayed on).  You can just mix starter and soymilk and put it
into a glass (not plastic) container and leave it in the oven for 7 - 10
hours or so.  Some electric ovens get that warm if the oven lightbulb
is left on overnight.

Alternatively, almost any warm place will do.

The culture will only thrive in a narrow temperature range, too cool and
it won't be active, too hot and it dies.