Before constructing a mannequin, you must decide on the look you desire. Do you need or want a full body form? Or is a torso adequate? Do you need a head and hands? Is realism important? Or is the mannequin merely a support for the costume, and should be as unobtrusive as possible? Are you creating a historical diorama? These are important questions for you to answer before you start measuring and cutting. Remember, the more realism you require, the more time must be devoted to making a mannequin for each costume. While a torso might take as little as 4 hours for an experienced mannequin maker to create, a realistic human figure can take more than two full-time weeks of work and may require more than one person.

The Padded Hanger
The simplest technique is the padded hanger. This can be a commercial hanger, padded out to provide some support to the garment. Or you can modify a commercial hanger (cutting off the hook and using fishing line to hang it). Testfabrics once sold quilted padded hanger torsos that could be modified with ruffles and other attachments. You can make your own custom padded hangers, which is how the Smithsonian's National Holocaust Museum hangs costumes. These have flat slotted metal hangers to which are attached quilted cotton body forms that pad out the hanging costumes. 

The Flat Cut-Out
The flat cut-out is the next step towards realism. I've seen these made of wood, plexiglas (clear acrylic sheet), cardboard (acid-free), coroplast (corrugated plastic), metal, polyethylene foam plank (Ethafoam), and foamcor.   

Generally, tailored garments do not look good on flat cut-out supports. The Rock and Roll Hall of Fame in Cleveland, Ohio displays fancy gowns on cut-out forms - making the gowns look ugly. The breasts aren't well supported and the gowns have stretched wider than they would have looked on the female singer who originally wore them. In this instance, the mannequin actively detracts from the display. Flat cut-outs work best if they are very narrow and then well padded out with polyethylene foam or polyester quilt batting - giving them a three-dimensional character.    

Making two cut-outs and spacing them 3 to 4 inches (7.5 to 10 cm) apart, helps give a start to the three dimensional character. However, the shoulders and breast plate need to slope inwards, so the boards are only about 2 inches (5 cm) apart at the shoulders. An example of this mannequin was published in Museum Mannequins. 

Sometimes flat cut-outs are hinged. This is like a full-size paper doll. I have seen these made of white cardboard as well as wood. The wood technique is fully described by Elmer Halvorson in AASLH Technical Leaflet 64. He finishes the body off with a papier mache torso, hands and head.

A buckram cast torso form (left) and Ethafoam carved torso form (right) sized for the same garment

Carved out torsos are usually made from polyethylene foam, the best known brand is Ethafoam. However, a cheaper alternative might be styrofoam plank, which is readily available in most parts of the world. (It might produce stearic acid if heated up. However, tests I've had running for over 20 years have shown this material to be remarkably stable in real life conditions.) Planks can be vertical or horizontal. They can be cut using Denis Larouche's silhouette technique, Curtis Peacock's detailed sculpting technique, or cut using the disc technique published by many authors. Ethafoam doesn't require any adhesive - it melds together with hot air from a paint stripping hot air gun. (Your bathroom hair drier is not hot enough.) Styrofoam does require an adhesive. Commercial styrofoam adhesives are nasty - full of fumes that test acidic. Hot glue doesn't work that well on styrofoam - it usually peels off. But double stick tape might work for your purposes. Or bamboo skewers to hold pieces together. (Bamboo skewers are great when trying to hold pieces of foam together.) 

Realism
Finally, there is realism. It can be suggested - as in paper hair or silver painted faces. Or it can mimic reality as much as possible. Here you need to be well aware of all details, because a slightly incorrect depiction is glaringly obvious to the museum visitor. Realism almost always requires casting from a real person. This means finding a person who looks like what you desire, who is willing to undergo the mental and physical torture of having face, hands and other body parts copied, and who will not balk on seeing their face in your exhibit. Or it requires the talents of an exceptional artist who can mold a face in clay or carve it out of a more rigid substance. Mannequin faces have been cast in polyester, plaster, a variety of modern art casting materials and wax. Colors have been tints in the wax or polyester and paints (applied with brush or spray gun). The creation of a realistic mannequin requires knowledge from theater props departments where casting and molding humans is more commonly practiced. And in the end, you might require a skilled artisan who can create magic.

What Effect Do You Desire?
So, as you think of your exhibit, think of what the overall desired effect should be. Do you want a basic support or a complicated realistic figure? Or something in between? Do you mind the headless horseman mannequin? Or are necks only necessary if you need a head to support a hat?

What Materials Do You Have?
Then think of the materials available to you. Do you have a ready supply of something that is acid-free or can be altered to become acid free? For example, wood can be covered in aluminum foil to block acids emitted by the wood as it ages.  

You need to consider the length of use for the mannequin. PVC tubing may create hydrochloric acid as it ages. Perhaps you should use acrylonitrile butadiene styrene (ABS) plumbing tubes - they are dark gray and more inert - rather than the white PVC. For a short weekend exhibit, the use of dress forms draped in quilt batting and covered in cotton stockinette might be adequate. (We did this for a 3-day exhibit of First Lady gowns in Alaska.) But if the exhibit must be up for years, then be more careful about the materials you choose for the mannequins you make.

What Versatility Do You Need?
Also consider the costume. How will you put it on and take it off the mannequin in the least stressful manner? Do you need to build the mannequin so that it comes apart in sections? Often there is a break at the waist and shoulders. Arms can be inserted into sleeves before the garment is put onto the torso. How will the sections attach? Do you need a metal catch plate device because these areas must support a lot of weight? Is velcro sufficient? Do you need an internal rod?

How will the mannequin attach to the exhibit? Placing attachments can be tricky, especially on a fully clothed realistic mannequin, such as a man wearing a suit with shoes and pants. If the shoes are an exhibit prop, then they can be damaged. But if they aren't then you need to find a way to support the whole figure without putting holes into the costume and accessories. Will there be a back wall, a roof, a floor for you to use to stabilize the figure? Remember, most mannequins need support at more than one point. Otherwise you might get whipping or leaning.

As you can see, there are many considerations that go into building mannequins for your exhibits.