In total, there is no sense to write about the grounds for the body, because it is better to show videos, but I'll give you a little hint for each one!
Muay Thai training - elbows
*****the elbow blows, it is important not just hitting same, around the elbow, you should concentrate on hitting by the protruding bone on the inside your arm when you bend it. Put your hand on the shoulder and you'll see it.
****When hit, try to keep the body lean forward slightly it will give a extra power blow (also applies normal punches;)
Muay Thai Training - Spin Back Fist
****Do not hit the back of your hand, it is painful. Use protruding phalanges of your hand.
****Blow bring forth from the shoulder not only with the same hand-it will be stronger.