I use a tongue depressor to mark my layers, but you don't have to. I just happen to have thousands of the little buggahs in hand, so they are handy. I will then measure the depth of sand, silt, and clay, as well as the total soil depth in the jar. These measurements are used to calculate the percentage of each soil component.
For example, the jar above shows 2 1/4 (2.25) inches of sand, 1/2 (0.5) inch of silt, and 1/3 (0.33) inches of clay, for a total of --------3.08 inches. Divide each particle depth by the total soil depth to get the percentages:
2.25 divided by 3.08 = 0.73 or 73% sand
0.5 divided by 3.08 = 0.16 or 16% silt
0.33 divided by 3.08 = 0.107 or 11% clay. (I rounded to the nearest whole number)