First, get something you can throw. Opponents crotch block, your hand, your head, your foot, your opponent's pec... Anything. Then, build momentum, and keep your elbow bent. Use a lot of joints in unison, but make sure they maximize your momentum. When you think you have enough, use shift+space and ungrab/regrab in the same frame until your ghost shows you hitting. You'll need a lot of speed, at least 35 to get a dismember with a hand. Heads have more mass, so they don't need as much speed, but they still need quite a bit.
A good way to break your own had off is to grab your hand, contract the wrist, elbow, and pec of the same side, then space until you grab yourself. Usually, that'll break your wrist off. Then you can ungrab and catch it with your other hand.
Here's two replays of basic skeets, showing two different ways to throw. You can also use your dismembered hand to grab your foot and practically kick your hand at uke, but that takes more skill.
If you make your hand grabbed while it's flying, it's more accurate, and sometimes will get you the dismember. Sometimes, staying a fist until you ghost into the opponent, then grabbing, will result in a DM. Just pracitice.
Have fun, happy skeeting.