Step1
Ask politely for permission. He needs to know he will be treated with respect throughout this process. Ask if you may have or make copies of any service papers or other records, those will give you valuable information to begin your research. Ask for permission to use a tape or video recorder. If he does not wish to talk, do not pester, respect his decision.
Step2
Before you start. Read as much as you can about the war in which your grandfather served. The rank-and-file soldier usually isn't concerned about the politics that caused the war, but it's good for you to understand why he had to go to war. Read as much as you can about what it was like for those who served.
For instance, when I was in school, I interviewed my grandfather who served with the 5th Regimental Combat Team in the Korean War. From my pre-interview research, I knew that the weather was extremely cold. Knowing that beforehand allowed me to ask intelligent questions. Grandpa says it was so cold,
Before you do the actual interview, ask what unit he was in. Then research as much as you can about what that unit was doing in the war.
Step3
Write a list of questions that you will take to the interview. Do not feel that you have to stick to these questions rigidly. Let him talk. Avoid questions about whether he has killed anyone and be very soft in your approach to anything dealing with actual combat. Ask about living conditions, what he did to pass time, etc.
Step4
Once you arrive for the interview, ask permission once again to record the conversation. Try not to place the recording device directly between you and him. He will be looking at you most of the time and having the recording device to the side will help him be more at ease.
Step5
Handle with care and be flexible. If you ask a question and he doesn't want to answer it, respect his wishes and move on to something else. Give him room to tell stories. When I talk to my grandfather about the war, he moves from story to story. You will get a lot of information from those stories.
Step6
Keep it ongoing. This can be something you talk about for years. Your grandpa will likely enjoy telling many of these stories. You'll hear a lot of culture shock stories: My wife's grandfather, for instance, has told me several about his service as a Navy Seebee in the Pacific during World War II, including one about a pet monkey the sailors had that was killed and eaten by natives.
The best way to get him to keep telling stories is if he knows you have a genuine, heartfelt interest and enjoy hearing them.