A roman tub is constructed on a frame using 2x lumber, plywood, waterproof membrane (polyethylene tub & shower seal or hot mop), cement board and tile. They are supported on the floor, so are not poured concrete which would be too heavy.
The tub is constructed much like a large custom shower pan except the waterproofing is extended upwards. The layers of a tub consist of framing, structural backer, membrane, backerboard, thin-set, tile and grout, sealer.
Construction of the waterproof membrane is critical, so you may want to get some professional help. At the very least, you can probalby get plans and specifications from a specialy tile supply store. You might also find information on liners and membranes at a professional plumbing supply store. I would start by checking at the tile and plumbing stores for details. This project is fairly demanding in terms of carpentry, plumbing and tile work and is not for the average DIYer. With some help you can certainly get it done. You may want to hire some construction assistance for the frame and plumbing and do the tile work yourself.