so far, i have only managed really basic kongs, but i became confident with them by repeatedly doing them over a low balance beam in a gym until I could do them well and confidently. i then went out to where i usually train and can now do them over slightly higher rails and walls quite happily.
Start off with standing kongs, position yourself a couple feet away from the obstacle, place your hands on it at the same time as jumping forwards. keep doing them until you are happy you can do them. then try it with a run-up. once you are comfortable with that, try a slightly higher obstacle. keep doing them until you are comfortable.
"dive over a small wall and kong the other side"
what you will want to do is find something like two low rails close together. simply pretend that it is something like a picnic table and place your hands on the second rail and perform a normal kong
and if you are worried about kneeing yourself, keep your legs tucked!!! really, really do. it hurts like a -blam!- hit my knee on sunday, still slightly sore now. so yeah, keep your legs straight to start the kong, and the tuck them around when your hands hit the obstacle.
hope this helps
^ please note i may be talking absoulte gibberish, in which case, please disregard all of the above