First, identify all of the stakeholders that will be impacted by your project. These are the people that will benefit from the success or failure of your project. This also includes people that provide input into, receive output from, or work the process, the process owner, etc.
Next, rate each according to their current attitude toward your project. Are they neutral? Slightly or actively supportive? Slightly or actively against it?
Then identify where you need them to be in order for the project to succeed. Do they need to move from actively against to neutral? From neutral to actively supporting?
Finally, develop plans and take action on those plans to shift these attitudes, or work around them if possible. This is the hard part.