It's almost impossible to provide a 'one size fits all' workflow for fitting mixed models because so much depends on:
1. Which JMP product are you using? There are big differences between JMP and JMP Pro in terms of capabilities for this broad family of modeling methods. As a former JMP senior systems engineer, my stock recommendation to experimenters and modelers is if Mixed Modeling is something you really depend on, then seriously consider JMP Pro because it has a much broader set of capabilities compared to JMP.
2. So much depends on the specific experimental design structure, definition of effects, and the model you are attempting to fit. There is a multitude of pathways wrt to experimental design structure, effect estimation, etc. which all influence specific selections within the JMP/JMP Pro workflow ecosystem.
The above may not help much...but without knowing which product you are using, more details wrt to the problem at hand, the specific experimental design, etc. If you haven't already a good place to start is the JMP online documentation wrt to Mixed Models found here: Mixed Model JMP Documentation
I also recommend taking a look at this JMP On Demand webinar presented by my former colleague @jiancao with some great explanations and examples: On Demand Mixed Modeling Webinar