You can either use the white root end of a vegetable that you have already cut, or buy a handful of new vegetables to use specifically for growing.
Simply place the white root end in a glass jar with a little water, and leave it in a sunny position. I keep mine in the kitchen window. The green leafy part of the plant will continue to shoot. When it’s time to cook, just snip off what you need from the green growth and leave the white root end in water to keep growing. Freshen up the water each week or so, and you’ll never have to buy them again.