How do I jungle?

Asked this a while back but noone replied. Think I should be more specific this time.

So here we go: How do I jungle?

I end up filling jungle more often than I'd like (read: more than 0%) so I need to know how to play jungle.

But every time I do, I end up not playing properly. Whether that's because I'm using the wrong champion or build (unlikely because mobafire), making the wrong decisions (clearing jungle camps vs ganking), or not ganking properly. I just can't do it. There's too much to watch out for. I can't just look at red bars and right click them when they're low like in other lanes.

So, how do I jungle?