After identification of the topic for the study, Search peer reviewed databases (like Emerald, Sage, Wiley, Springer, Science Direct, Taylor and Francis) for latest research (preferably 1 Year Old) on the topic.
Review the Proposed Limitations and Future Research Directions
Identify any new constructs/variables proposed that may be added to the model. New and original contribution stems from adding new constructs to the model, new study setting is replication and not contribution.
Do not just blindly add the variables, also search if the relationships make sense in the context of your study and can you explain the proposed relationship.
Do not take variables from one single study, another scholar may have also read the same paper and may propose a similar study, this will waste a lot of your time.
Finally, discuss your model with your supervisor or subject expert.