Looking old in a few minutes!

1- Build a chimney and draw the bricks with a sharp tool...

2- With sandpaper, sand secerely all the angles so that it doesn't look new anymore. Then  paint it with a red tone so that it looks like a new chimney...

3. Now apply thin layers of darker colors such as brown and black. Use a sponge to spread the color on some areas only. The darker the chimney gets, the better it looks. Add some yellow and some green to the basic dark colour. When there's too much color, jut wash it away with water.(I use acrylics)

After a few minutes, the chimney starts looking like the one on the picture on my laptop.(see picture 2 in the background, the sample picture)No the painting is fresh so it shines but when it's dry, it'll look mat. Tomorrow, I'll check it and add some color or sand some areas.
Now put the chimney on the roof and glue it in its place.

Now ageing the roof was not that easy as no error was allowed since the roof is covered with...paper and it won't appreciate getting wet. So I used a watercolor crayon and very little water. I mainly underlined the structure and washed the colour away, I added a little green next to the gutter.
Then I painted the parts around the window so that it may look like rusted metal.

With a different light, this is the result:

Maybe you'll think the patina isn't enough but I have checked a few pictures of zinc roofs and there aren't many signs to show zinc is  getting old.