To undertake improvement activity in your existing business processes in a systematic way using Lean Six Sigma, you need to employ the useful framework of DMAIC:
Define: Projects start with a problem that needs solving. Make sure everyone involved knows their role, why you're doing the project and what you're trying to achieve with the project.
Measure: The work you've done in the Define stage is based on what you think the problem is. During the Measure stage you need to clarify things by seeing how the work gets done and how well.
Analyse: Now you know what's happening, it's time to find out why, but don't jump to conclusions. Manage by fact to check out the possible causes and get to the root cause.
Improve: Okay, you know about the process and the problem. Now, in the Improve stage, you need to find a way to address the root cause, so come up with some ideas, select the best one and test it out.
Control: You need to implement the solution, and ensure you achieve and hold the gain you're looking for. Putting a control plan in place is vital to ensure that the process is carried out consistently.