Fold your fabric into four layers as suggested. Use two rulers. With your 24"x6" ruler, make sure the fold next to your belly is straight along the edge of the ruler; then check and make sure the other fold is straight along another line on the ruler. Adjust and readjust until the folds are parallel.
Take a second ruler and measure the desired width, first at the belly fold and then slide this second ruler along the edge of the 24" ruler and make sure the measurement is the same at the other fold.
When cutting then, secure the 24" ruler with your hand closer to the belly, cut for a short distance, stop, move your hand toward the other end of the ruler, cut to the end.
When you have to butt two pieces of your binding together, cut your ends at an angle and stitch. This will distribute the thickness when applying the binding to the edge of your project.
Hope this helps. Suzanne