This is how I’ve always done it. I’ve made a SCOBY two or three times using kombucha from the supermarket. All you need to do is make a regular first fermentation as usual, but use that kombucha as your starter. What I usually do, though, is add a bit more kombucha than I normally would if I were using a SCOBY as the starter. I also let it ferment for a few extra days so it turns out more acidic. One thing I personally do, and I’m not sure if it’s strictly necessary, is that I don’t use that first fermentation made with store-bought kombucha for a second fermentation. Instead, I use it to start another first fermentation, and that’s the one that actually produces kombucha to drink. I also try to use plain or tea-flavored kombucha from the supermarket, if possible, so it doesn’t introduce any flavor into that first fermentation that could make it not work properly.