The basic flow of a buy-in can vary depending on the circumstances; if the person who's buy in you need is someone deeply involved in the situation, there's one approach.
If the person who's buy-in you need is actually so close to the core problem that they will feel under attack as a 'directly responsible person" then you need a different apporoach.
And if the person is an outsider but still someone who's buy-in is needed -- for example, a bank manager in a situation where financing will be a necessary element for the solution to succeed - they may be best approached with a different form of buy-in, too. These are the nuances of the process, but they are not trivial.
The Basic Flow of a Buy-In
1. Gain agreement on a clear definition of the symptoms to be resolved or eliminated. This step alone can be time consuming and create headaches in all participants, but as you focus on this it'll be clear to just what extent many managers attempt to launch solutions, solutions that might involve scores of hundreds of people and with 7-figure or 8-figure budgets, without there ever being a clear definition of what symptoms are supposed to be resolved. Another way of expressing this is, they launch the solution without ever defining what "success" looks like. Failure is pretty much a certainty.
2. Use the Current Reality Tree to illustrate how an unresolved conflict (the core problem) leads cause-effect to the symptoms. A sneaky tactic is to guide the participants to identify even more problems arising from the core problem. Buy-in becomes rock-solid.
3. Expose the Cloud to highlight the assumptions that render the conflict unresolve-able ... then use the Evaporating Cloud technique to show how a challenge to the assumptions can open the door to a solution that is "outside the box." Show how a certain direction to a solution will eliminate the conflict and generate a win/win solution.
4. Walk through the Future Reality Tree, from the bottom-up i.e. from the "good idea" emerging from the Cloud, step by step until one by one all the symptoms have inevitably been eliminated or minimized as a result of the many elements of the solution being put into place. Let the participant(s) improve on it themselves as much as possible. Be explicit as to how every one of the symptoms that effect them is clearly dealt with.
5. Once the "trunk" of the FRT is solid i.e. the connection between the "good idea" and the demise of the symptoms has been fleshed-out with all the necessary elements of the solution and a significant improvement to the symptoms will be inevitable ... then use the Future Reality Tree to explore all the potential negative side-effects of the solution, and plan those elements of the solution needed to prevent the negatives from becoming reality.
6. The added elements in the Future Reality Tree, those elements that will have to be added to convert the "good idea" into a comprehensive "good solution," now need to be examined to determine which ones can be put into place only after overcoming substantial obstacles. The Prerequisite Tree is then used to diagram the obstacles and, more importantly, plot a "roadmap" through, round or over them. This deals with the last "yes, but ..." aspects of the Layers of Resistance.