Vacuum forming is a process by which plastic is heated, then pressed over a form while a vacuum sucks it down. The plastic hardens over the form, reproducing its shape. The forms for this process are called plugs or bucks. They can be made of any material. Medium density fiberboard (MDF) is an ideal material for heavy duty plugs. They take longer to make than other materials, but are very durable and last for years.
Cut out pieces of MDF to build up a rough shape of the finished object. For example, if your finished object is a pyramid, you would begin by making a step pyramid.
Glue the various layers together. Use clamps to hold them tight while the glue dries.
Use saws and carving tools to refine the plug into the finished shape. Don't worry about adding a lot of detail, it will not be captured in the plastic.
Drill holes in sunken areas to ensure that the plastic gets sucked down (for example in the eye sockets of a face shape).
Cut out the various pieces of the plug. This technique works best for geometric shapes, but not well for curves and organic forms.
Trim the edges to whatever angles are needed to form a seamless joint. For example, a pyramid made of four triangles would need to have their edges cut at a 76-degree angle. The more complex the shape, the harder this cutting is to accomplish.
Glue the pieces together, and further secure them with wood screws. If the plug requires larger screws, drill pilot holes first to prevent the MDF from splitting.
Fill in the screwdriver hole in each screw depression with wood putty. Also use wood putty in any seams that are not perfect.
Sand the wood putty smooth.
Round the edges if desired.
Vacuum form plugs must never have edges that curve under the mould, or the plastic will have to be cut off. For example, a bowl turned upside-down makes a good plug, while the plastic would wrap around a bowl turned upright, and lock in place.