Answer:
Glass itself is made mostly from silica (sand), sodium oxide (from soda ash), calcium oxide (from limestone), and small amounts of minor ingredients. The minor ingredients give the glass its colour - for example a little bit of gold makes the glass red.
The liquid glass is poured onto a table of molten tin to cool and sliced into saleable pieces
As for turning the glass into a stained glass design, pieces are cut out of the sheets of glass and assembled into a pattern of various colors. The glass is held together by lead strips called "came" then soldered into a finished piece.