Basically you go into the editor and through every environment in TMUF to try and reach the decorations outside the building area. You can go to next environment when all 4 wheels of your car touch the specified area, touching water or its ground does not count.

The speedrun would start when you enter the editor on the first environment, and it'll end when your 4 wheels touch the deco of the final environment. You can also place blocks however you like (no blockmixing mods allowed).