There are several different ways to go about creating prescription maps. First you need to decide what you want to use as the basis for the prescription to be created. This will need to be a single layer within the software. It can be Yield data from one year, normalized yield data, soil type maps, soil test results, or a map that you have custom drawn zones into.
Once the base map is decided on, the next step is to determine how you will come up with the rates to be applied in the different areas of the field.