Place the tracing paper over the drawing. Tape the drawing under the tracing paper so it does not move, making sure it is as symmetrically centered on the tracing paper as possible. This is not only aesthetically pleasing but keeps yourself from getting confused and feeling like you messed up the proportions of the drawing later on.
Trace the border of the drawing with the ruler. If there is no border, use the T-square to make a rectangle around the drawing. This is where putting it symmetrically in the middle would have its merits. Divide the height and length of the rectangle into equal fractions on all four sides. The easiest fraction to use is to divide the drawing into one-inch sections. For example, if your drawing is five inches by seven inches, then there are 35 one-inch by one-inch squares dividing the drawing.
Trace the drawing onto the tracing paper as well. You should now have your original drawing on the tracing paper with the lines cutting the drawing into those inch-by-inch squares.
Measure the area you want to draw the mural on with the measuring tape. Following the previous example, if you want to transpose the five-inch by seven-inch drawing onto a 15-foot by 21-foot wall, you will make every inch-by-inch box of your drawing equate to three-foot by three-foot squares on the wall. The mathematical equation for enlarging the drawing is an x-inch by y-inch drawing being enlarged to fit an a-feet by b-feet wall equals each one-inch by one-inch square turning into an (a/x) by (b/y) foot square. When you divide the drawing into these boxes it becomes easier to redraw the drawing--enlarged--on the wall. Taking the drawing sections one at a time and noting the distances between the lines of the drawing and the lines of your boxes helps to increase the proportional accuracy of your mural.