How was glass made in medieval times?

1st Answer:

They weren't made. They hadn't been invented yet.

2nd Answer:

Optical lenses go back almost to prehistory. We have record of Nero using one to watch games. Some of the important laws governing their shape were known to Arab scientists in the 10th century. Glasses were being made in Europe by the end of the 13th century, and there are numerous medieval pictures of people using them.

According to articles I have read, the lenses for medieval glasses were produced on pole lathes. There are links below to the history sections of articles on lenses and glasses. There is also a link to an article on pole lathes.