I raise my fruitflies this way.
1 cup mashed potatoes mix, a tablspoon of sugar and a pinch of yeast. Mix the ingredients, add some vinager (helps to prevents mold) and water. Mix. You might have to add more water or potatoe mix if it is too dry or wet. I myself make it on the dry side.
I then use 20 oz. soda bottles for the cultures. I put some small holes at the top of the bottle, thumbtack. I cut the bottle in half. Place some of the mix in the bottom of the bottle. Tear a piece of paper towel and place in bottle. Cut a small reverse "v" into the top piece. This is so you can place the top into the bottom section a little. I then duct tape the pieces together again. Use a funnel to place some fruitflies from another culter into the new culture. 1 cup of mix is good for 3 20 oz. bottles.