I have done this following Lum Eisenman's instructions, and had good
results. Here is the link:

http://home.att.net/~lumeisenman

Read Chapter 20 for sparkling wines.

The amount of carbonation you get does depend on the amount of sugar
you add, IF you successfully re-start fermentation. Just adding sugar,
or yeast, or both may not be adequate. I add the sugar to the base
wine, then re-start fermentation gradually from a yeast starter,
following the procedure for a 'stuck fermentation'.

Lum recommends 1 1/2 oz. of sugar per gallon. For a six-gallon batch,
that's 9 oz. of sugar; I think a cup of sugar is around 7 oz., so that
works out to around 1 1/4 cups. That would at least get you into the
reasonable range.

When the second fermentation is active, bottle in champagne-style
bottles using crown caps (metal bottle caps). These are much easier to
pop off during the disgorging process. I use the plastic champagne
stoppers and wire hoods after disgorging.

Fermentation in the bottle takes several weeks, at least. I usually
start the "riddling" phase after 3 months or more, as the contact time
with the yeast is supposed to be beneficial. If you add any sweetener
during the disgorging process, I'd recommend adding some sorbate to the
sugar syrup, to prevent yet another fermentation. The whole process
usually takes me 6 to 12 months, but it is very satisfying to do the
disgorging, and wire down the plastic stoppers on the finished product.


Happy fermenting --

Doug