Step 1: Stay Calm
It helps to convince yourself that you are telling the truth to begin with. You can not be lying if you don't think you're lying. You might be delusional, but that doesn't matter since you have convinced yourself you are telling the truth.
Step 2: Speak in generalities
It's good to offer as few details as possible. Only give details if asked for them. Details can be used to unravel lies.