Upload a picture of a clarinet to your computer. Microsoft Office offers free clip art on its website, as does Clipart.com. Look for one that is cartoonish and has easily recognizable features unique to the clarinet, such as distinctive keys, mouthpiece and bell.
Find a suitable piece of cardboard to create the clarinet. If the clarinet is a stage prop, you will need a piece of cardboard large enough to be visible from the stage, like the side of an appliance box. If you are making the clarinet to decorate a room, the inside top of a large pizza box will work well. Pin the cardboard to a wall with pushpins.
Project the picture onto the cardboard. Print the picture of the clarinet on an overhead projector sheet before projecting. Move the projector mirror up or down to adjust the image to the desired size. Alternatively, project the image onto the cardboard by using PowerPoint or a similar program. Open the program and add the image of the clarinet into a Presentation document. Connect your computer to a projector and adjust the size of the picture in the document to fit the clarinet image to the cardboard.
Draw an outline of the clarinet on the cardboard by tracing the projected image. Stand to one side or crouch beneath the image so that you do not block the image source while drawing. Use a pencil at first then go back over it with black permanent marker. You can trace as many or as few details from the clarinet as you choose.
Cut out the picture of the clarinet, using a sharp craft knife or box cutter. Use the blade to cut around the borders of the clarinet. With thicker cardboard do not cut all the way through the first time; doing so may cause the cardboard to tear. Cut around the clarinet a second time following the same cutting line and do it a third time if needed.
Paint the clarinet with the acrylic paints. Although clarinets are traditionally black, with silver keys, you may also select colors that work well with other decor in the room or on the set. Do not apply the paint heavily or it will saturate the cardboard. Allow the paint dry then add another layer if needed. Let the paint dry completely and then you will have a cardboard clarinet to use for whatever purpose you choose.