It’s easy. And it works. Put a teaspoon in the left or right pocket of your pants. Whenever you catch yourself thinking a negative thought, move it over to the other pocket. Do this without exception. Over time you will notice that you kick out the negative thought before it has a chance to be fully formulated in your mind. The first couple of days you will constantly have to move it back and forth between your pockets. After a month, sometimes less, it becomes a habit and you develop the reflex to dismiss negative thoughts, no sweat. If after a while they do creep back in, put the spoon back in your pocket and scoop them away.
PS Yes, seriously, this works like a charm. Are you drowned in negative thinking? Try it.