For the overtones put your tongue somewhere between an L and an R sound. Then move around your lips to create different o shapes while humming. Move around your tongue slightly in different ways as well. At some point you'll accidently create an overtone for a brief moment. Then see if you can recreate that. Slowly you'll be able to consistently hit the overtones and from there it's smooth sailing.

For the throat singing it's the exact same technique beat boxers use to create the deep bass. Just try to make a exagurated sigh. Do so until you accidently do throat singing for a brief second, then see if you can recreate that for a longer duration.

It's all about trial and error! It's really not as hard to learn as it might seem :)