You can learn faster, protect yourself, and preserve your surroundings if you set up a knife throwing range. First you need a safe area behind your range. A tall and solid fence with nothing vulnerable behind is good. You want a clear line of sight on both sides to insure nobody walks into your line of fire. Next you need your primary backstop that you expect the knife to hit fairly often when you miss your target. A full sheet (8 foot long) of 3/8 or 1/2 inch plywood is good. It is a good idea if you stand this upright and tilt it towards you. I would tie the top of mine to the fence and let it tilt forward by about 45 degrees. What this does is to deflect misses down to the ground instead of back at you. To protect your knife when this happens put a bunch of collapsed cardboard boxes and stuff like old carpet under the overhang of your backstop. Extend this out in front a ways as well.
You want a target that a knife will stick into, but will minimize bounce back when you don't get the rotation right. I like to collapse boxes and tie about 6 to 8 layers of cardboard together. It works to lace them together with wire or twine. A nicer way is to get some Dupont 77 spray adhesive and laminate them. A good target is 18 to 24 inches wide and 24 to 36 inches tall. You don't want it too heavy since that will encourage bounce back. I hang the target on about 18 inches of cord from the top of my forward tilted backstop. Since the target is a bit soft, a bit flexible, hanging free, and not too heavy it will reduce bounce back when you under rotate your throw.
The last step is to take some masking tape and mark your rotation distances. I like to throw from the blade so the first thing I do is step back about 5 feet and experiment with where I need to stand to get a perfect stick with my most natural throw (my 1/2 rotation distance). When I find it, I mark it with some tape on the ground. I experiment and find how much further forward than my mark I can stand and stick the knife with a bit faster rotation and how much further back I can stand and get it to stick with a bit slower rotation. Now I go back to my next natural distance (when I throw from the blade end this is my 1+1/2 turn distance). Since my first mark was at 1/2 turn my 1+1/2 turn distance is around 3x as far. So if my first mark was at 4 feet my next mark would be 12 feet. (Throwing from the handle my first mark might have been 8 feet and my second mark 16). Now I go back to that distance and see if my natural throw will stick in the target. You can do a little adjusting and your style will develop, but this is the general dimensional guide. When you get it working go out and place a third mark at your third natural distance (for my example this would be 20 feet). My old range had marks all the way out to 50 feet.
For practical purposes you really want to get solid at your first mark. If I was in a pinch that is about the only range I would be comfortable throwing a knife. Also it is tough to estimate distances and you really want to be able to spot your natural distance and work from it. My next priority is to be able to figure when I am a little closer or a little farther out than my natural throw mark and be able to overspin or underspin my throw to stick my blade from where I am standing. I designed the target and backstop to protect myself while working at this close range. When your close range work is solid move back and work from the second mark. Here you start to find accuracy a challenge. When you get good there practice shifting between your first and second marks. When that is solid try ranging between your first through 4th marks. If your technique is solid your marks will let you stick the blade whenever you have solid distance information.
Consistent technique is critical. I always hold the blade the same and take the same type of step when I throw (particularly in the beginning). I work a lot at my natural distances and work on a "normal" arm and wrist swing. This establishes a baseline. Primarily when I want a faster rotation (for a slightly shorter range) I make my step a little shorter and give the throw slightly more wrist action) and when I want want slower rotation I take a longer step and hold my wrist a bit stiffer. I try not to vary anything else. Extra variables add confusion and slow down learning.
I bought a set of COLT CT19, made by United Cutelary... 