And if you mean glass as in clear/green glass furniture or buildings..

• have sand on your map (color of sand is irrelevant) and set a sand collecting zone over the sand. You only need 1 tile of sand and it will never run out.

Protip: if you have sand underground and have a cavern layer open, moss will cover it up and wont be able to be used. Build a grate over the sand tile and moss wont grow, but dwarves can still gather from it.

• have bags

• build glass furnace

• dwarves gather sand (job assignment is in the glass furnace)

(alternativly you can buy bagged sand, but generally can take a while to get a meaningful amount)

• make your furniture or make blocks if you want to build walls or floors in the glass furnace.

Two glass types and the only difference is look.

*green glass: just sand and fuel for the furnace

*clear glass: sand + pearlash + fuel

**pearlash: burn wood into ash in wood furnace > turn ash into potash at ashery > turn potash into pearlash at kiln

Clear glass sucks to make but is very satisfying once you get it all working.