Designing
To correctly design the Holyfield's house, make sure you add in "Cookies and Treats," "Holly Items" and "Walls with Christmas Theme." The "Cookies and Treats" should be placed on the shelves on the left wall. The "Holly Items" should be draped over the window on the left wall and the cabinet on the back wall. The "Walls with Christmas Theme" are green with a few pictures dotted around. See Resources for a full image of the layout of the Holyfield's house. This layout will give you a Gold Medal finish if you complete the room in time.
Coffee/Eggnog
Watch the meters above your worker's heads at the bottom of the screen. These represent their overall energy level, which affects the speed at which they work. If you don't keep them topped up with coffee or eggnog, they will go on break and waste a lot of valuable time. The aim is to keep the meters above the workers' heads in the yellow. A full meter means a better worker, and a lower meter means there is a chance the workers will get injured and require first aid.
Other Worker Tips
The workers all have on different colored outfits. This is their color, and if they are set to work on an object glowing in the same color, they will work much more efficiently. Workers do not lose energy and complete the task much quicker when the color is matched. Use multiple workers for bigger tasks, so they are completed more quickly. After day 5, you can use the "Carol-oke" machine. Assign a worker to the machine, and they will sing a Christmas song to the other workers, granting them a speed bonus.
Other Tips
After you have the correct design, the only thing required to complete level 18 is to build efficiently. Workers occasionally ask for tools, which can be found in the toolbox in the upper right hand section of the screen. The toolbox opens when a worker requests a tool. Find it, and click and drag it to the worker. If you don't do this quickly enough, the worker will get it very slowly. Workers also occasionally leave trash, shown by a trash can icon beside the worker. Grab the bag of trash and place it in the trash can in the bottom right of the screen.