Mixed methods-grounded theory (MM-GT) has emerged as a promising methodology that intersects the value of mixed methods with rigorous qualitative design. However, recent reviews have found that MM-GT empirical studies tend to lack procedural details. The purpose of this article is to apply the "best practices" for conducting MM-GT in a study designed to develop and then test a theoretical model for how undergraduate engineering students develop interest in the engineering PhD. This study contributes to the field of mixed methods research by (a) illustrating best practices for MM-GT, (b) providing an MM-GT scale development example, (c) demonstrating how an MM-GT scale could potentially bypass exploratory factor analysis and proceed directly to confirmatory factor analysis for testing psychometric properties, and showing how a joint display for data collection planning can be used to strengthen integration in an instrument development study.