Wizard of Oz approaches

Step-by-step guide
PREPARATION

1. Review scope and clarify prototyping questions: What do you want to learn or explore? Look at your starting point and consider if and how you will bring previous knowledge into the room (for example, as a research wall or as key insights). Do you want to test the whole or just a part of the interface? What are the tasks that you expect the chosen user to do? How detailed do you need or want to get? Make a list of the tasks you want to test for later. 
2. Identify participants: Based on your research question, define criteria for selecting suitable test subjects. Use sampling techniques to select your test users and consider including internal experts or external agencies for recruitment. 
3. Prepare scenarios and create interface elements: Use digital service or investigative rehearsal sessions to generate a set of key scenarios. Then use suitable techniques to prepare the key elements the users will interact with, e.g. leveraging cardboard prototyping, paper prototyping, wireframing, or sketching in code.
4. Rig ’em, assign roles, and practice: Rig all the relevant parts of the service or system to allow the “wizards” to control the interaction and appropriately react to the actions of the user. Then split your team to take on the roles of operators (“wizards”) and observers. Allow the wizards to practice until you achieve the intended experience.
5. Set up the test space: Set up your test space. You might want to establish a video link or a mirror wall so the wizard is hidden from the user while still being able to observe the user. 

Step-by-step guide
USE/RESEARCH

1. Test the prototype: Conduct your test. Introduce the project and the context of your prototype and ask the user to perform a certain task from a selected scenario. As the user starts to use the prototype, the operator simulates the operation of backstage processes, devices, or the environment by manipulating the objects and environment behind the scenes.
2. Keep a list of bugs, insights, and ideas, and review issues: During the test the observers will record their observations and create a list of the issues that they discover. After each testing session take a few moments to reflect on what worked, what didn’t work, what you would like to change or try next. Briefly discuss the issues you discovered and prioritize them. 
3. Revise and iterate: Check off the task or scenario that has just been simulated and quickly decide which one you want to try next. Revise the reactions of the “wizard” and consider changes to the respective elements if necessary. Then go again. At the end of your testing session, reveal the wizards and do a final debrief with the users.