Creating stained-glass windows beings with an artist's design on cardboard, which the glazier traces onto paper and scales to life size. This drawing is cut into pieces to make the templates for the glass cutter. The glazier also notes the colour of each of the tesserae that will make up the window to ensure it recreates the original painting as faithfully as possible. Once cut, they are put into a network of lead lines to hold them together.

Then, they are put into a metal frame, which goes into the window. At the Sagrada Família, stained-glass artist and glazier Joan Vila-Grau oversees the whole process: from design to selection of the colour of each piece to the location of the lead lines.