The technician selects a lens blank with the add and a base curve as close as possible to the prescribed power. Another curve is ground on the back of the lens to reach the exact level of the prescribed power.The prescriptions can be ground in negative powers for nearsighted, or positive powers for farsighted .
The lab technician will choose the blank lens that is close to the required prescription thickness. Then they will use a machine to grind it to the required curve. FInally, they cut the lens to fit the frame.
The procedure of making eyeglasses is quite complicated and can't be explained fully in several word. You can have a look at this:
https://www.firmoo.com/help-c-24.shtml. The part of "How to make eyewear" gives you detailed information. Hope these can help you.