Dear Toby:
The soil moisture regimes used in the 2nd Ed. of U.S. Soil Taxonomy are defined by measureable criteria in Chapter 3. The SOP could be written to list the steps needed to accomplish the measurements. As far as steps for using Soil Taxonomy, you may find it in the new Illustrated Guide to Soil Taxonomy. Both documents can be found on the USDA-NRCS "soils" web site at http://www.nrcs.usda.gov/wps/portal/nrcs/main/soils/survey/class/