Most modern guitar builders are now using what is known as an external mold
for building their guitars.
These days there are many types of commercially made forms available.
Here are some them.
This one is particularly unique because of the way the spreaders are made.
This type of spreader adds uniform support against the sides
keeping them square to the top and back plates while building.
You can also quickly open the form and remove the guitar during the building process
just by removing the pins on the ends of the form.
Here is another commercial form made from plywood.
It also has a way to quickly open the form but you would have to make your own spreaders.
Here is an adjustable form that allows you to use it for several different sized models.
You can place it over your plans and using a square,
adjust the support blocks around the edges to match your guitar shape.
Here is a classical guitar form
that has had an area cut out so that the Spanish heel can be used while building the guitar.
Here is an example of some spreaders that can be made
to hold the sides against the edge of the form while building.
In the old days we had to make them ourselves
and I am now going to show you how to do this.
However, I must warn you, if you are going to make just one form,
by the time you add in the materials, your time and all the mess you are going to make,
it is better to just buy a commercially made form.