sounds easy and perfect but its not. You cannot predict everything around you. For example if you visualise yourself at a public speech, you visualise yourself stand in front of people. You are ready for it, but what happens if you have to change a bit what you have been prepared for?
Visualise: 1. what you want it to be 2. what you dont want it to be 3. what it could be so you are ready for whatever comes your way. ready for any small thing that *could* go wrong
theres plenty of techniques, but basically you have to study the real scenario and elements that your visualisation will be based on, then you start by adding those elements to your mental practice until you perceive them to be real ( taste, smell, etc ) and you will have to create a 'movie clip' that you loop over and over until it feels absolutely real in your mental practice.