Potassium metabisulfite is one of the most important winemaking compounds. It is an antioxidant and bactericide that releases sulfur dioxide into wine must. Use 1/4 teaspoon per five gallons to add 50 ppm. Or, mix 1/4 pound in 1 quart of water to make a stock solution; 1 teaspoon of stock solution in 1 gallon of must yields 50 ppm sulfur dioxide.
I use K2S03 to halt fermentation when fruit wines taste good to me. When the alcohol is above 10% and the flavor is good, it is time to move the wine to a secondary fermenter and add the potassium metabisulfite. Mix it in well and let it sit for a week, then filter and bottle.
I used this on apple cider. I filled four 6 gallon carboys with cider and had two days until I could actually make the batches of wine with them. The Pot. Meta. worked will, but I recommend to put it into the carboy early so it mixes well. I had not added it to the first one until it was nearly full and it started working on me by the next morning. I quadruple dosed it and mixed it in well, which shut down the fermentation. Later the juice worked well in fermentation so no issues.
Pretty straight forward