Practice, practice, practice. Yes, today you will practice the scenario to the point of an option is being offered. At this point, you can practice stopping the conversation and asking for time to consider the option that is presented. This is better than refusing the option or accepting it before you are ready. It is a reminder that you control the process. You started it you can stop it.
Let's look at your goal and what you have done so far. The original goal was to have the flexibility to work from a coffee shop, from home or even while traveling. The plan was to negotiate this new schedule with your boss. The steps we've covered so far include:
Day 1 - Know your contribution to the company.
Day 2 - Know the values of the other person. (In this case, your boss)
Day 3 - Understand the company policy regarding flexible schedules.
Day 4 - Know how you are perceived.
Day 5 - Sketch out a scenario to practice and find a practice partner.
Day 6 - Practice the scenario you sketched out. Ask yourself if you were balanced in your approach. Were you realistic but not apologetic or boastful?
Day 7 - Practice again this time using open body language.
Day 8 - Determine what your Plan B might be.