A method and apparatus for forming a mast assembly upon a floor of a drilling rig has a support structure, a first carriage assembly affixed to one side of the support structure and a second carriage assembly affixed to an opposite side of the support structure. Each of the first and second carriage assemblies are connectable to a section of the mast assembly. Drive cylinders are associated with the first and second carriage assemblies so as to cause the section of the mast assembly to move upwardly relative to the support structure. While the section of the mast assembly is retained in an elevated position, another section of the mast assembly can be placed within the support structure and joined to the elevated mast section. Additional mast sections can be connected together in a similar manner.