What is the best way to do a Drug Intervention?

My father Inlaw has been using drugs for 35 years now and my husband and I have found Meth in his garage and meth pipes also. My sister Inlaw wants all of us to get together and plan and Intervention on him. But im worried about doing it right. What should we say and how do we begin? We just want him to get help and to go rehab or detox willingly. Please give me good advice so that I can be helpful. Thanks alot!!!