It's a simple case of finding your distance and then being consistent in your throw. You throw like a pitcher throws a baseball...start over your right shoulder, step into it with your left foot, let if fly and follow through. After a few tries, you will find your distance and it will start sticking.
The best targets, IMHO, are tree slabs. When you see a public works crew taking down a large tree, you will see that they have slabbed the trunk into large "cookie" round slabs, sometimes as wide as 3 feet. If you ask nice, they will often give you one or two or more for free because that means they don't have to pick them up and haul them to the dump. These are the best throwing targets because you are throwing into "end grain". They don't split and shatter. They will last for years.
If you can't find a crew taking down a tree, go find a local firewood cutter and ask if he can get you one.