The simplest chimney design is for one that protrudes from the roof of a structure. Even so, that sort has two minor variants, depending on whether the flue protrudes from the peak of the roof (below left chimney) or from the sloped roof deck (below right chimney).

That position determines the sort of mounting tab and shape of the lower edge in the design. These instructions are for a chimney on the roof peak. They can be adapted for a non-peak chimney.

Three measurements are needed; sometimes you just have to decide what looks best:

1. Width of the Side of the Chimney (a)
2. Width of the Front / Back of the Chimney (b)
3. Height of the Chimney at the Peak of the Roof (c)

Draw a series of rectangles about twice the height of the chimney (c) and widths b, a, b, a, b (as below). Then overlay a profile of the roof peak from your design (in green below).

Make a couple of rectangles that will be the chimney top closure (size a x b) and place them on your drawing above the appropriate chimney side. Now draw a cut line around the shape, including the chimney topper and tabs below. Draw fold lines between the sides, at the junction of the roof topper squares, and horizontal across the b sides at the level where the roof profile intersects the side lines. Remove the guide shapes and lines, but leave the topper squares. Make the topper square lines non-cut, and fill the shape with a gradient. Fill the large shape with a brick pattern, appropriately scaled.