The old glasses you got should be for nearsighted and WITHOUT astigmatism. The more nearsighted the better...but people with serious nearsightedness are also more likely to have astigmatism as well.

Dissemble the glasses. Glass lens are better but plastic ones should work. Be careful not to scratch the lens.

Put double sided tape around the edge of the lens. WIPE THE LENSES!! Then stick the two lens together in () direction by wrapping a black, firm plastic film.

My piece of plastic film came from this plastic shopping bag :P

Make a case to hold the lenses. There're various ways to make the case but I found these materials at home and made do of them: hard black foam and an iPad box with its plastic insert.

Cut a hole that fits the shape and size of the lenses in the foam. If the foam is thick enough, one piece is enough. If not, make more pieces. How to decide on thickness?

Placing the lenses too far away: notice the black edge on the bottom? That's the edge of the lenses

Then cover the foam with some plastic or cardboard. Keep the case as light as possible. You don't want your phone to tilt after mounting the lens on. This picture shows the front of the lens.

For the back, cut a hole in the cover so that it doesn't cover the phone camera. Make sure there's space to stick the mount on. There're many ways to make the mounting part but I used iPad box insert.

The corners of the inset in the iPad box fit the corner of the iPhone perfectly. Cut two of the corners and stick them together to make the mount.

Cut the edges around the corners and stick them together to form a clip that is just as thick as the iPhone.

Place a ruler in front of & parallel to the phone. Phone should be in landscape. Take note of the two ends shown on screen: it's actually 10 & 20 but snapguide trimmed photos to squares

Hold lens in front of camera at max distance from phone. Move left & right to make sure the length augmented on both sides are the same. 9.3 & 20.7 ie 0.7 augmented on both sides for real

Then mark where the mounting part should be placed on the lens case and stick it on.

This is how it looks like. Kinda cartoon-ish, I think. Only works landscape - well, this is wide angle.

Sitting about 3 feet away from the piano) Normal: 16 keys here (22 keys for real)

From exactly the same spot with DIY wide angle lens: 18 keys here (28 keys for real)

Top: normal Bottom: wide angle

Top: normal Bottom: wide angle