Steps to creating a machine learning model

Suppose you work for a furniture and home goods eCommerce company. The company is dependent on ad campaigns for a large chunk of revenue. As a part of the data science team, you’d like to build a machine learning model to predict if a new user will convert through an ad campaign. Describe how you would approach building, validating, and testing the model?

Note that this question is open-ended and can have multiple solutions. The idea is to practice framing the answer logically and to keep it high level enough for an interview, but detailed enough to let the interviewer know you understand the complexities of modeling.