Cut a cardboard to about twice the size of the lighting equipment you will use for it. For amateur users, this light source may be any practical lightbulb or a construction light on a stand. These are typically used for home moviemaking. For professional film shoots, most production lights are bigger, especially the stronger ones that can readily illuminate large spaces. As a basic guide, you can use a cardboard size of 25 inches-by-25 inches. This can ensure you that the gobo can effectively cover your practical light source.
Draw your desired shapes within your cardboard frame. These shapes are similar to drafting or drawing templates where there are holes with specific shapes meant for drafting. However, in the case of a gobo, it is not used for tracing a shape using pen and paper. It alters the light that falls onto a scene being shot. And so, these shapes should show how your desired set can be shown on your shot. For instance, you can make holes with shapes that look like leaves. The gobo makes it look like these leaves come from a nearby window and it blocks the sunlight to cast shadows on your interior shot. You can also make a gobo with a shape like a window. Although a window is not shown on the shot, it will seem like there is one nearby because the gobo placed over your main source of light casts a window-shaped shadow onto the shot.
Cut the shapes drawn on the cardboard frame using a cutter. For your safety, choose a flat surface and preferably a cutting mat for this process.
Paint the cardboard frame black on both sides. This follows the concept that black absorbs light and it doesn't reflect light that strikes on it. Your gobo allows the light to pass through its holes without altering the light's original direction.
Secure the contours of your gobo by surrounding it with a gaffer's tape or adding any hard material that can protect it from quickly suffering from wear and tear. Cover these areas at least three times with gaffer's tape. This is a basic adhesive tape used in production work. Note that a gobo is typically attached to the front of the light source using a clamp or by simply using a gaffer's tape to temporarily adhere it to the light source. Don't use other kinds of tape or other similar adhesive products as they may react to the extreme heat from the light source, especially when using popular production lights that can significantly heat up after prolonged use.