A logic model is a graphic illustration of the relationship between your theoretical foundation, methodology, data collection, and analysis. Think of it as your purpose (i.e. research question, theory, etc.), inputs (i.e. activities, data collection), and outputs (i.e. outcomes, data analysis, triangulation, etc.).

Assists you in planning and explaining your study through the model creation process
Assists others in quickly understanding how your research study will be deployed.

Creating your logic model should be done in parallel with drafting Chapter 3 (Theory and Methodology). It should be revisited and revised periodically prior to and after your preliminary exam as well as during your final dissertation draft when you determine the ultimately data analysis and triangulation elements.

Identify the Research Question(s)
Determine the Theoretical Foundation(s)
What are the inputs? Determine the Key Data Collection Sources
What are the outputs? -Determine the Analysis Process
Identify how the data will be triangulated
Create a Logic Model Outline of the relationship of the above-mentioned elements.
Convert the outline to a visual depiction of your research study

See some samples in our Theory and Methodology workshop presentation

Here is a google search link