Chase,

If possible, please refer to the SpectreRF User's Guide. The User's Guide has a section called "Creating and Using Transmitter J-Models" which describes in detail how to create a large signal model of a transmitter. The thing to remember about the J-model is that it is memoryless. Thus, you should model your baseband filters separately. The J-model will capture AM/AM, AM/PM, PM/AM, and PM/PM conversions. When you extract the J-model, be sure to sweep the input signal past the amplitude you expect to see. The model is good at interpolation but not extrapolation.