Possibly the Romans already used this method of glass production. It was certainly known in the early Middle Ages, and cylinder glass remained the most common type of window glass into the twentieth century.
The method is as follows (see image):
1. A blob of molten glass is taken from the furnace with a blowing pipe.
2. The blob is inflated.
3. The resulting balloon is blown into a mould, or swung around until it attains a cylinder shape.
4. The ends of the cylinder are either cut off, or, to save material, are reheated and opened up.
5. The cylinder is cut open lengthwise, placed in a special furnace and folded open.
Naturally, as with all manual crafts, each master had his own tricks of the trade, and thus the details of the process rather vary.