You rub two (thick) pieces of glass together.
One is stationary, and the other is rubbed over it in all directions, and there is a grinding paste between them. The bottom piece will become convex, and the top piece will become concave. You start with a coarse grain, and as you approach the form you want, you use finer and finer paste, and polish at the end with paris red.
These will be almost perfect spherical surfaces, given a bit of practice.
The cause of this is, that the outer edge of the lower piece is more in contact with the (grinding) surface, so there more material is taken off than at the center. Also the outer edge gets more pressure, as the contact surface is smaller.
For the upper (moving) piece it works just the other way around, so this will be the concave surface.
Amateur astronomers used to grind their own concave mirrors this way (and some probably still do). For mirrors the quality of the glass is not important (as it will be surface silvered), for lenses you need good optical glass.