Here's a basic walk-through I put together explaining how to bring out the overtones. I ran by each step really quickly so if you have specific questions feel free to email me.
Some things that I left out.
-when making the "L" shape with your tongue, think of it as if you're pronouncing "ul" As you lift up the back it will naturally making an "ee" sound without you making it the "ee" shape with your lips.
- the BIG challenge is going to be to hold the pitch you're singing in tune while listening for the pitch of the overtones. Don't worry about that at first. You will want to work on getting the overtones to really ring, THEN you can work on sustaining the pitch that you're singing in tune.