If you wanted to build the Collesium or the Eiffel Tower what would you start with?

For a project, my partner and i have to build a famous structure around the world.....we were planning to do the Roman Collesium or the Eiffel Tower. But to begin the project, what woukd you build it with? How do you measure the length, width and everything?? I really need some help. Thankyou :)