If you have an Astatic mic it's easy to hear when it's properly cupped. You can get a great distorted tone through a tube amp. You can also feel the pressure pulling on your hands on draw notes. I seal my hands around the harp and mic, but not to my face.
Practice with just a harp (no mic). When you have a great seal you can play a low note and a higher note will sound (the air flowing through a hole that is not sealed by your mouth, but is sealed by your hands on the inside of the cup). Your hands should feel the pull from draw notes. Now add your mic and try it.
Hand position - My hands are small so I don't hold my hands exactly as shown in much of the instructional material. Just try different positions until you feel comfortable and can get a good seal. If you've blowing harp for several hours you need to do something that feels good.