Hi Pavi,

You can use databases like Google Scholar, Scopus, Ebsco etc. and do a thorough literature review. Identify the gaps and future works that can be done on employee performance. once you found a research gap, then you can use previous scales/questionnaires of renowned authors for your model or create a survey if you are developing something new and get it verified by a few researchers and industry experts- that each of the item you have considered for employee performance is relevant or not. Then get the questionnaire filled and test your hypothesis.

Good Luck